100HP is always good for stage 1 Pokemon. You can endure most attacks with such high hp. However, as a EX this is only average. No weakness is always good, but then no resistance for a flying type can be rather odd. However, at least you aren't weak to fire or psychic, or something bad like that. 1 retreat cost is standard, allowing easy switches.
Dive is just an average attack that fills up the time if you're not doing too well with energy. It's just there for the sake of allowing Altaria to do something. Even against stage 1, Dive will take 3 turns to kill something (everyone will try to avoid 40 Hp Pokemon now.)
Dragon Mist is a super powered Swift attack. Its energy cost is very reasonable. We got used to dragon types having a combination of energy already, and we'll just have to make do with it. The bad part is that it's only 70 and not 80. 80 is the magical number for all thsoe stage 1 non-ex Pokemon running around. 70 makes it short by 10, and this will give them the time to suicide blow against you.
It's not a bad Pokemon, but I won't rely on it in serious play.