1) I again echo everyone’s comment’s: She’s not the one wearing the dress. I have a very particular style and HATE a lot of dress styles. But if I’m not the one wearing it, it doesn’t really matter now does it? If you had asked for her opinion and she said she didn’t think it was flattering to you, that would be one thing. And if she had only said it once, that would have been one thing.
To continue to bring it up – does she really think you’d go “Oh! Okay let me change my mind!” It’s a dress, but it’s also MORE than a dress. It’s an emotional experience – finding “the one” that you can see yourself wearing on such a special day in your life. So when she makes those comments, it hurts more because the experience and decision you have made to pick this dress are made to seem insignificant, irrelevant, untrue, and without meaning.
2) I imagine your Mother-In-Law is married (or has been) – therefore she already had her turn for her wedding and making that decision.
3) I also imagine she is not paying for the gown, or contributing towards it. No $$$, no say. But you should still be able to pick a style you want, regardless.
4) I echo the idea to tell her “I’ve already chosen the dress. I love it! I do not want to speak about the dress again.”
My mom will often make comments about things that bother me, and says I should just say “Okay mom”. I think in your case, you can essentially say “Okay” to acknowledge you’ve heard what she has said (I don’t know, does she really think your ears are blocked from hearing when she has said this?), and say you do not want to talk about the dress again, unless it is positive. She might twist your words and say “Oh well I just think X, I’m just telling you what I think, it’s not negative.”
You can say “Well, I know you really care about me, and I am sure you are very excited for the wedding, but these comments have made me uncomfortable and I do not want to have it brought up whenever I talk about the wedding or the dress. Please respect this.”
5a) SHE’S NOT EVEN YOUR OWN MOTHER!!!! Not sure how close you are but it’s so rude. Mom’s can often get away with more but even a mother would realize to knock it off (unless she was just so self-involved/careless).
5b) WHO CARES WHAT SHE WANTS, HER DAUGHTER WAS MARRIED ONE WAY AND YOU ARE NOT HER DAUGHTER. You’re family but you don’t have to “be” exactly how her daughter is. That’s a part of what makes you, you.
6) YOU ALREADY BOUGHT THE DRESS. THE DECISION HAS BEEN MADE. END OF DISCUSSION.
Edit: I posted this and saw additional comments you made, wanted to add something more.