The problem with Transform Mew is that it lacks the DW ability that made Ditto so viable. If you're coming in on something with boosts already, you're going to ave to spend a turn transforming, which will probably kill you. If you're using Transform on something without any boosts, then the set's really only viable if you're copying something that the rest of their team is weak to.
On the subject of Transform, tho. I forget the details, since I never use DW Ditto, but it copies all stats/boosts except HP, right? Assuming I'm not making that up, imagine an Eviolite Dusclops with tons of HP sitting there like a boss. And yes, I'm aware of the holes in that strategy. But it's just. so. BULKY. haha
Edit: Oh yeah, my point. Mew has better things to do than play dress-up imo