Mewtwo doesn't die too quickly unless you make mistakes. I've done very well in teams vs other pros with M2. Granted in some situations I've had to resort back to Marth over M2 for overall effectiveness, but it still holds true that M2 is good at being annoying, holding stocks, saving teammates, getting grab finishers, edgeguards, and setting up for combos/smashes with neutral airs.
I've played M2+Jiggs with The King, M2+ ICs/Sheik with Wobbles, M2+Peach with Cort, and M2+Marth/Falco/Sheik with Forward. You only really hit a wall as M2 if there is a Fox on their team because it is harder to perform your role. If you think that you're "offensive" in nature, you can still advance the other team because you can use throws to get people off of you then send to your teammate and tech chase the guy you just knocked away, throw fully charged shadowballs into them and lock them down when the situation arises.
I've played M2+Jiggs with The King, M2+ ICs/Sheik with Wobbles, M2+Peach with Cort, and M2+Marth/Falco/Sheik with Forward. You only really hit a wall as M2 if there is a Fox on their team because it is harder to perform your role. If you think that you're "offensive" in nature, you can still advance the other team because you can use throws to get people off of you then send to your teammate and tech chase the guy you just knocked away, throw fully charged shadowballs into them and lock them down when the situation arises.