Big no-no. How rude and attention seeking.
I know my big mouth, and I would have to say something about it, but do what you think is right. Silence on your part, and letting her go ahead and make a total fool out of herself might be better.
Maybe there are extenuating circumstances, she had the dress and can't afford to buy another, but doubtful something like that is happening.
Gee, I see this all the time, the MOH picking same colors as the bride. The MOH is the luckiest person connected with the wedding, she has only one wedding dress code rule to follow, to call the bride's Mom and make sure they do not pick the same dress. Otherwise, MOH gets to wear whatever dress and color she wants. And if it were me, boy if it were me I would definitely take advantage of that.