A woman who wants to ruin your wedding any way she can?

Princess Caroline of Monaco's daughter, Gabrielle, about the same age, did this, too, recently.  So your SD is in good company.  Lol.

One day sd will look back and wonder why BM let her look like an idiot in the photos....... at least you will look stunning and BM can't do a thing about that 

There's still time to get a headband or some cute clips to hide the hack job!

BM took it upon herself to "trim" YSD's hair one year right before we were scheduled to take Christmas card photos. She has no training and it was truly the worst haircut I've ever seen. More like a mullet than anything else. She tried to get to OSD too, but the girl has always been a princess about how she looks so she threw a fit and BM gave up. I was not happy, but H was absolutely livid. Not so much about the photos but just that BM would so cruelly send her daughter into the world looking so ridiculous just to spite us. He took her to my stylist to get it fixed (because we knew the shitstorm that would ensue if I took her) and she was able to cut it into a decent pixie cut. Which didn't suit YSD at all, but at least it wasn't a crappy haircut. 

Incidentally, this was not the only time she intentionally sabotaged the girls' appearance. Terrible ill-fitting clothes, awful dye jobs, cheap makeup and low quality skin care, orange fake tans, and bad manicures were a regular occurrence for many years until they were old enough to take a stand. And I won't even go into how she allowed them to spiral into obesity completely unmitigated to the point that she would change doctors every single year whenever one told her that they needed to improve their nutrition and start getting some exercise. 

My armchair theory is that she can't stand them being prettier than her. She also can't stand them being happier than her, more educated than her, having better taste than her... So you can see the pattern. She wants them to stay with her and wallow in her white trash misery forever. 

can't tell you how many time my skids came over with a crappy hair cut.

GWR wanted a mow hawk one time and DH took him to get it done.  Came back two days later with his head shaved.  Beaver shaved it because "she" didn't like it.  Not GWR....Beaver

SD has somewhat thin hair and is about 40 pounds over weight.  She was growning her hair out and HATES bangs.  Beaver took her and got her a blunt cut bob with bangs.  SD walked in and DH asked WTF happened to your hair.  Beaver took her to some fly by night hair cut place and told them she wanted this mess.  SD looked like she was a lego person.  Her hair was cut the same as lego hair.  It was AWFULL. The only thing the my hairdresser could say was "Well at least they cut it somewhat straight".  Took SD years to grow out that mess.

Could this be a ploy by BM? Sure, but why do it now when there is time to fix it, or to buy a wig, or figure out a new way to style it?

Could it be that SD and BM got into a spat, with SD having a tantrum about wanting to cut her hair and BM just let her - with the consequence being that SD had to live with whatever mess she cut?

Or it's entirely possible that BM didn't even consider your wedding and she has a passive parenting style where she lets the kids do what they want with their hair, up to and including cutting it themselves.

I know HCBMs gonna be high conflict, but either she's really dumb by doing it so far away from the wedding or your wedding wasn't what prompted SD's haircut. You know your BM better than I do. If it's within her character to do something that humiliates the kids to spite her ex, then you can assume it's the former. But, if she seems to not give a damn about your existence or finds other ways to hurt your FH, assume the latter.

buy her a cute pretty headband or tiara and she'll be beautiful. 

This is why we kept our wedding a secret.  Killjoy didn't know about it until we were getting in the car.

Never let a BM know your plans for anything.