Donovan Mitchell is good. He has outstanding quickness that gives him very high level shot creating ability. He is probably better at that than either Brown or Tatum. He is a very intriguing player. I like him slightly better than Beal for example.
But I can't see the Celtics getting involved in this with Brown (which it is likely what it would take). Why do a lateral trade to fix a problem that doesn't really exist. Yes, Mitchell would bring additional end of the game shot creation, something we would benefit from, but as others have said, we give up size, defense, and other intangibles.