I agree that Dawn is more battle-oriented so I think it's okay for her to win against May in a battle. And I guess since May has Glaceon, I think that would at least be something she has gotten out of going to Sinnoh.
The way I see it, if Dawn loses to May, it's a good opportunity for Dawn to learn from May, who, as we know, is masterful at appeals more than anything. She's got some battle skills, as we all know, but where she shines has been in the appeal rounds.
May would be more than willing to help Dawn improve her appeals. I've mentioned this before: Dawn and May could easily work with the mentor/big sister kind of relationship. I can see May giving Dawn a big piece of advice on appeals, Dawn of course taking it to heart, and later, putting it into practice. Afterwards, we won't see Dawn failing in the Appeal rounds quite so badly.