"Could it be that exH gets MIL to brush dd hair and she complains about knots? I can imagine that might be a reason why she decided to go for it. I used to keep suggesting a hair cut to one of mine for years because of the hassle of brushing it."
My grandfather had my waist-length/never been cut hair shorn by his barber to a shaggy ear length effort at a girl's hair cut, when I was 6 and staying with my grandparents for the summer holidays from school (yep; all 6 weeks of them!) without my parents permission almost 40 years ago. When they turned up to collect me at the start of September, he presented my mother (his daughter) with a plastic bag that held the two bunches which had literally been cut off just above their bobbles.
And my mother went ballistic.
Years later, I asked him why he'd done it and he said that he literally couldn't stand idly by and watch me melt-down in absolute terror every time my grandmother or oldest brother (big age gap, he was 20-ish at the time and was raised by my grandparents from birth, pretty much) attempted to brush my hair - so he did something about it. What I never told any of them, but often wonder whether my grandfather knew anyway, was that my terror was because my mother took to beating me with the 1970s wooden hairbrush she used every time I squirmed or cried out when she encountered a pug (and she would yank the brush through regardless - it's a wonder I wasn't bloody bald because of her!), when I was little more than a baby (my hair has a natural wave to it and is very fine, so... pug/knot central!).
Okay, yes; it was a bad haircut, and I definitely looked like a boy - but to this day, I have never had hair past my shoulders since my grandfather stepped in and said "enough!" (My mother still used him for the free child care, though...)
Having said that, however, my ex-MIL had my then-toddler son's baby curls (first haircut) shorn into the "neater" short-back-and-sides which she preferred when he was under-3. His hair was never knotted, he never cried when it was brushed, and he left my house one day looking like a toddler - and returned looking like a miniature adult. I've never forgiven her for that, and he's 17 now - and insists on a Peaky Blinders style cut (his hair = his choice, I just pay for it). That really was a power-play on her part, and even her son admitted it was when I cried over it. Yes, it's "just hair" but she stole his first hair-cut from me and from her own son, too (we were together at the time). There've been other efforts at undermining me over the years, but she's reaping the consequences now, because my son refuses to see her unless he's forced into it by his Dad - and then he'll turn into a grumpy teen, so she doesn't get the benefit of his actual personality at all. Which is a shame, but... it's down to her. Not me.
So, I can see it from two out of the three sides. I just hope I will never be that MIL or grandmother who dares to think that they have the control over the child's hair, rather than the woman who actually carried, gave birth to and is raising them.
So yeah; power-play on the ex-MIL's part, OP, and I'm sorry but it'll ramp up the older your daughter gets (taking her to have her ears pierced, without checking with you - who'll have to do the aftercare - for example; my mother did that with my then 9-year-old daughter and it wasn't fun, because my daughter hadn't banked on it hurting and they could only do one correctly, with the other at a definite angle!). Unless you lay out some ground rules about both parents having to be consulted regarding changes to the children's appearances, of course, and you make it clear that she had her time to be a mother; now that's your job.