a solution for all mewtwo's starting out: wave dash more.
you should also use less teleports. it has enough vulnerable frames to constantly put you in situations where you would get punished.
-Don't roll. If you must, teleport.
its quite the other way around i believe.
lots of characters have attacks that stun your shield, which they would use to follow up into combos. examples include fox's dair to shine/grab, falco's laser to grab, etc. in such scenarios, instead of getting grabbed, roll is your best option. the shield stun prevents you from wavedashing or teleporting, however shield stun hardly affects your roll.
teleport should be used at minimal. teleport often helps you escape aerial combos. but teleport has a delay that is long enough for you to get hit. teleport only comes with few invincible frames, other frames, you are vulnerable.
you will often find yourself with enough time to execute teleport, but not enough time to actually escape their combos. experience helps you determine when is a proper time to use teleport.
the thing about rolling is, it ensures that you will get out of a situation. it doesnt alwyas garentee safety after the roll, but at least you are out of the danger you were in. in fact, if you are rolling outside of the shield stun, your safety is almost absolute because your enemy also has cooldown to slow him down. on top of all, mewtwo's roll is one of the longer rolls.
so in short, if your shield is getting hammered, roll away.
AHHH LONG POST