Sonic Boom
@JohanSSB4 Twitter
From looks alone, AG is like Mega Evolution, but that's just because of the idea that it's a "Mega Evolution" for Ash to keep him from using a Mega Evolution during Kalos.
Mechanically, though? Extremely different and really not the same as a ME. Any Pokemon that can change forms simply has different triggers, but they're not all under the same mechanic to trigger them all. MEs occur when tapping a stone. AG triggers when it gets cocky from defeating one opponent.
Mechanically, Battle Bonding to Ash-Greninja is also a mandatory form shift; once Greninja KOs something, you cannot stop it from going into the Ash state. Mega Stones are at least manually controllable, so you can decide when you want to Mega something, if you even want to at all, as Mega Manectric plays in VGC can attest.