Sasquatches, Jiang'shi...
What's your team? If you use Haggar or Tron, then I think it's doable, but Dormammu's primary problem is that he is ridiculously slow. He has absolutely no hope of keeping competent top tier users off of him, and he starts matches at a disadvantage because his backwards walk is very slow, so Wolverine/Wesker/whatever gets to start the match on top of him.
Dormammu is not a dominant zoner in this game; he gets smashed hard by all bullet characters and has to rush them. If your bullet-using opponent uses Haggar assist, the matchup is extremely difficult for Dormammu on point because he's not fast enough to bait those assists and punish them.
Dormammu is requires assists to function, but he also is a high-risk character that pays dearly for mistakes. He needs meter to cover those mistakes. On point, you often play without the meter you need to make those decisions. Without meter, your opponent doesn't have to be as scared of Dormammu. If you've watched F. Champ play, his Dormammu gets wrecked where many others would not simply because his opponents know he won't risk burning a meter to punish their mistakes because he's saving for Dark Phoenix.
Putting him second or third resolved all of these problems. I'm not saying you can't win with him on point, but you are starting your fight going uphill. If you can survive the free opening mix-up your opponent gets on you, you are still pretty strong, but still not as strong as you would be with a battery helping you out. In UMvC3, these problems will become even more glaring, because you can no longer hide behind invincible assists if you want to use him on point, and Flame Carpet got a huge nerf.
I think I posted the theoryfighter for the new direction I plan to take Dormammu on on here; I'm not sure if you read it, but I think it's a better way to use him, and will compliment the UMvC3 changes well.