Explain why you love it
Think about why you love the dress so much. Is it the fit? The fabric? The silhouette? Think about what it is you love about the gown. Do you love the way it elongates your waist or does the color flatter your skin tone? Have a few compliments in your back pocket in case mom has some less-than-kind feedback. You can counter what she says, for example, if her reaction is "this silhouette isn't what I was thinking for you" reply with something specific like, "I love the way it contours my body, I think really flatters my figure." Focus on how you feel versus telling your mom that her feelings are wrong.

Try on something she picked out
Chances are your mom is going to pick something for you to try on that you know you're not going to like. She's only eager to help so I recommend just doing it to please her. She probably won't pressure you to do anything further if you tell her it doesn't feel like you. It will make her happy to see you in a gown she picked out, so throw it on just for fun, it's not worth fighting over.