Hmm. After Parental Bond and before STAB, Return hits as a 153 BP move. Crush Claw has a 50% chance of dropping enemy Def on the first hit; in that case, it hits as a (75+75*0.5*1.5)=131.25 BP move, or a 112.5 BP move if not. Decent drop in power either way, but you've got an overall 75% chance of dropping their Def at least one stage for the next hit and a 25% chance of dropping it two stages. Dropping enemy Def isn't too common of a strategy, but I could see it being useful to muscle past specific checks.
Still, Power-Up Punch strikes me as a far more reliable and more permanent method of increasing damage done, despite its lower power. Let's see - over the course of two turns, using Power-Up Punch followed by Return on a neutral target, Kangaskhan will hit for the equivalent of 529 BP. With two consecutive Crush Claws, failing all of the first three Def drops (1/8 chance) means you'd hit for the equivalent of 337.5 BP, and succeeding on all three means you'd hit for the equivalent of (1.5*(75+75*0.5*1.5+75*2+75*0.5*2.5))=562.5 BP. That's really not a great improvement... If you miss either of the first two drops, which you will 75% of the time, you'll do less damage overall, and on average, you'll give them as big of a Def drop as you'd get an Atk gain by using Power-Up Punch, except the Atk gain is much more useful.
You do get to essentially combine two moves into one moveslot by using Crush Claw, but it really doesn't strike me as an improvement. Power-Up Punch is too good.
But seeing as mega gengar is faster, can mega kangaskan survive a single focus blast to guarantee you can start mud slapping his accuracy away? Maybe someone should calc it?
An uninvested Mega Kangaskhan takes 353 damage minimum from Modest Mega Gengar's Focus Blast and has 351 HP. So no.