Yes, I would say Mawile is worth training only if you use Mega Mawile. The reason I say this is because regular Mawile has a measly 380 base stat total. You aren't going to do much with that. Mega Mawile however, even though it has only 480 base stat total, it has the ability Huge Power. This doubles the attack stat which makes it extremely useful. Mega Mawile also has good defenses of 125 and 95 respectively. Overall, if you use the Mega-evolved version, it can be a useful pokémon. The only worries are its 50 base Hp and Spd. The speed issue can be solved by having a trick room user though. Hope I helped!
