Mewtwo can punish small things, but the margin is a lot less forgiving. Often times, you'll even need to just make calls to punish them. Sometimes it takes a preemptive wavedash or an air tight pivot to get the punish. Most of the time, you're not going to get the same opportunities that Marth gets, but Mewtwo has a way creating different and weirder opportunities to punish. It usually revolves around how you shield and wavedash into them. You can often force errors due to his weird height and get shield grabs, or mess people's timing up with his awkward tilt range. Unfortunately, the trade off is that you don't get to hard punish anyone most of the time. You just nickle and dime them until you get your up throw. Unlike Sheik and Falcon, they get big money from their grab game and it is a guaranteed KO on most of the cast at higher percentages.