Mega Drain is not STAB on Venomoth as it isn't on any game, and the low power won't help either. It is not super effective or weak against Alakazam either (Which is okay).
But summing this all up, with Alakazam's bad Sp. Defence and Venomoth's so-so Sp. Attack, Mega Drain is not as effective as other moves like Sludge Bomb, and against an Alakazam when both Pokemon are same level, this move's damage should be around 30 at the most. Without EVs, this attack will do very little damage, therefore a bad choice for a Venomoth.