I often wonder that too. Another questiont that pops to mind is, how do so many people in these HCBM's lives not see the crazy and fall for the "sweet, nice" persona that put forth. 

Meth Mouth = 5ft4in, brown straggly hair that is mostly gray, grayish skin due to drug/alcohol abuse and lack of proper nutrition. Blue eyes that are pretty much sunken in and blood shot. Crooked nose from when she drove off a cliff and stick skinny.
I posted a photo on here a long time ago and people said it was sad that you could tell she was pretty at some point. She was very good looking back before all the drugs, but I figure now her outsides match her detestable insides.

Satan looks like she's gotten straight off the slab at the morgue.  Pale, fat, bloated, dry dead hair combed straight back, white face powder, dead fish eyes.  She wears baggy clothes that resemble a shroud.

BM here looks pretty normal, I can see how people buy her nonsense. 

Our BM was short, like me, about 5'2".  Dark brown hair and eyes.  Her face was average but she had a wonderful tan back in the day when that was fashionable and great legs.  I had neither a tan or great legs but i had bigger you-know-whats and i was prettier.  Lol.

Our BM has bleach blonde hair, a big ballish nose, no boobs and chiclett perfect teeth but with a strained gritted teeth kind of smile. She's average height, weight but has zero sense of style. Everytime (which is once a year) we see her, same shorts, same flip flops and random shoulder cut out shirts. 

Mine used to be quite overweight, but had gastric bypass and lost a lot. She then began lifting at the gym and is SWOLE now (think brick truck, beyond regular muscle definition). Add Botox, fake boobs, skin surgery, eyelash extensions, full body wax, and you have her.  Add a deep voice with a thick Eastern Euro accent. 
She posts selfies of herself in lingerie on Instagram. She's 50. 

She got her name by, to me, looking like a Troll. some have said she looks like a long-haired gorilla with lipstick.

She is short, at 5'3" (I am 5'5", and DH is 5'10") round at 220 lbs. Shes always been very heavy and very mean. She would attack Dh when they were married, and he described her as a raging hippo. She has muddy green eyes, longish curly hair that she will dye a dark red, or a strawberry blonde. Its always self-done and from a box, so it looks patchy. She prides herself on being able to do her makeup and hair really well. She used to work at Macys makeup counters. I try to stay away from pink and purple lipstick because thats what she wears. Her chin is a butt-chin.

Ive seen photos of her back in the day when in her 20's and 30's. Big boobs. Now they fall to her knees. 

Some folks think she is really nice and all about God, from her FakeBook postings. She sais "please" and "TY", so some folks think she is just a down-to-earth nice girl, some folks think she is just a sweet "little" down-home girl who likes country music because she posts how much she loves merle haggard and garth brooks.

Those who actually KNOW who she is - we know shes a mean, trashy, trampy, sleezy sicko, and she likes bragging about her 3-somes. Not to judge but thats not the only thing she brags about...

She used to work with Autistic children for the local school system. Now they know her to be an opportunistic liar, working the system.

BM here is 5'4" - 5'5" and weighs at least 300 pounds...she has lost weight since being married to DH. She usually has poorly dyed blonde hair with at least one inch of dark roots. She has taken to wearing blond hair extensions. She has also recently added fake eyelashes that she does not know how to apply correctly, so she usually leaves a gap with almost no eyelashes on both sides of her eyelid. She wears a lot of make-up, but her skin still looks sallow all the time. She also looks haggard most of the time. She either dresses in early 2000-era peasant wear or mom-wear, both of which make her look much older than she is. Finally, one of her front teeth is a snaggle tooth. It is slightly crooked, slightly gray, and there is a black spot on her gums above it. When I see her, she also always have an expression of "woman on the verge", barely hiding her simmering rage. When she is around other parents, she takes on this weird fake air, as if she is somehow important and people should be listening to her. From the sound of her discussions with MIL and BIL, she always sounds hysterical when she is speaking with them, yet, they still believe that a lot of what she is saying is true. Which is weird to me, because they should know DH better than that (he's incredibly soft-spoken, conflict avoidant, and lets both of them bulldoze him all the them, yet, they believe he is "at war" with BM and spends all of his time antagonizing and badmouthing her), but you can't fix stupid. 

Looks like your ordinary 5' 11.75" corn fed, all american soccer mom and that's the trick of it.   Has large boobs but extremely large hips.  Excellent breeder.  Peggy Hill sized feet.  Auburn hair, blue eyes, straightened teeth ala braces paid for by Chef in part (They married and she still had them going into a retainer)., very fair skin with freckles.  Looks a lot like OSS (Pumpkinhead) when not wearing makeup.

Chef is 5' 8" so probably came to eye level with her boobs when she wore heels and that explains a lot.

Fake Church Lady who plagiarized her way through school and was banged by more guys than a turnstile (just like her BM, Battleaxe Galactica)   Worked for CPS (insert gasp here) for awhile, then a waitress for a local casino  trying to hide income and now counsels foster parents on how to parent. 

I've actually had quite a few people ask me what BM looks like, expecting her to be drop dead gorgeous with the crap that DH put up with and how entitled and conceited she acts and then they're surprised that she's completely average and imo a bit strange looking. Not gross or anything, but kind of alien like. I actually had the exact same reaction the first time I saw a picture of her - I was expecting her to be alot prettier from the stories I'd heard but she's just meh. I also assumed she was much younger, but she's actually older than DH.

https://www.google.com/search?q=picture+of+child+catcher+chitty+chitty&t...

BM looks like Gemma Collins. 

My DH definitely has a type.  When we were young, BM and I looked similar.  We are both curvier with bigger chests.  We both have dark hair, light skin, and dark blue eyes.

When we were younger, BM was probably prettier than I was. She had not aged well.  She is heavy now (I am also thick) but she is much bigger than me.  She has no teeth at all.  In fact, she refers to herself as the Hoover vacuum on social media.  *bad*  She also has very long dark hair that she pulls up in a greasy ponytail. She smokes like a chimney.  She really let herself go, which is sad because she was very pretty one.

I always imagine them to all look similar to my SS's HCBM because she just embodies high conflict with a nasty 'RBF' disposition even on her most fixed up days. 

My DH's ex has always looked like the suburban soccermom next door straight out of central casting.  Never lost the baby weight after 3 kids but not really fat, curly hair in a short "mom cut", no makeup, mom jeans and a cardigan, if she is "dressed up" it is in a long skirt and very modest neckline, and never a heel over an inch or so.  Look up "nice middle class mother" in the dictionary and there is a picture of her.  You would never know that it hid a black heart determined to PAS the kids and take DH for every dime plus some.  

We are complete opposites. I could not have children so I remained the same size as I was in my 20's.  Infertility genuinely sucks but it does have at least one consolation prize!  And I like fashion and makeup/hair/skincare so I look a lot younger than she does even though I am only 5 years her junior.  She used to make a really big deal about that and call me really nasty names to people at the skids' school.  I always wondered if they believed that I was a "home wrecker" just on her word or not but I stopped caring about a year into the whole thing.  Not a lot I could do about it anyway!