When you send out regular Mawile, its Intimidate activates and the opponent's Attack falls. When it's Mega Evolved, it loses the intimidate ability and gains Huge Power.
However, the opponent's attack drop remains after the Mega Evolution.
The thing that changes is that if you switch Mawile out and then switch back in, it won't lower the opponent's attack (it would if it still had Intimidate) because that ability is gone.
Hopefully this answers your question.