Jumping in late, as usual......but my thoughts are that it is acceptable to ask that a child doesn't wear something that inhibits their independence. I wanted to add, however, that dresses aren't the best idea also, certainly for the potty training child, as :
a. they have to lift them UP as opposed to down like trousers and leggings, a totally different manipulation (which tried in a hurry can end in an accident)
b. they trail in the toilet even if a child manages to get the dress up.
You could try these "skort" things which go down rather than up??
I think the idea is not only to foster independence but to alleviate frustrations.
A child that can't get their clothes off easily for the potty becomes frustrated perhaps?
I try to get around it by having a "weekend drawer" and a "school drawer" in the school drawer we have the plain t-shirts, easy pull up trousers or leggings and the weekend stuff is more glam and includes dresses. My dd ALWAYS without fail goes for dresses on the weekend (which is fine as I am around to help), but, to the moment (knock on wood

), we haven't had an issue with the desire for dresses for school.