NTA - but I personally believe that there is a much better way to handle this situation, and I’m just going to assume this is not the first time situations like this have happened.

I think even though she is your “step” parent, you still have to respect a person even if you don’t like them. From the way you described your responses, it sounds a bit immature and disrespectful. I agree that your stepmother is not being the nicest and most thoughtful person. Maybe try to be more subtle next time?

I’m not saying I know what I should do since I don’t know your family well, but from a third person’s perspective, I probably would say something along the lines of “Thank you for considering the possible choices of these pretty dresses, but I have been to these events before and I know my body better so would you mind if you take a look at the ones I choose first before giving an opinion? If you don’t want to buy them then I can get some elsewhere with my own money. I really appreciate your gesture.” if I were in this circumstance.