You're not wearing that outfit

You're not wearing that outfit -- Modern Family

be not doing sth mean:

If you want someone to stop doing something, saying "you’re not doing something" might seem a bit rude and straightforward.
要求某人不要做某事或终止做某事时，可以使用，此种表达方式比较粗鲁。

You could, in my opinion it sounds like you’re upset or mad about the outfit or that you’re mocking the person
So it has a negative nuance but “don’t wear that outfit” is more neutral and versatile

A friend could say, 'you are NOT wearing that outfit!' It would be an "order" meant to help, as ín...the wearer doesn't look good in it, so they should wear something else. It's a bit aggressive but between close friends (or a wife to hubby) it could be ok. It would not work between less close contacts as it would probably come off rude. Also, a husband saying this to his wife should tread very carefully. 😂

'Don't wear that outfit' is still something I'd use only between close contacts, but it's more neutral in tone.