He's not dumb in an overexaggerating way. Scizor is the one that's overexaggeratingly dumb (But also a jokester). The way Mewtwo is dumb is that he, like the Deoxys from the Meteor cave has the guts to tell the team face to face that they're gonna destroy them that way he looks intimidating.
Yes, Scizor likes to crack jokes and stuff like that but he is very clumsy and often forgets what he is doing. He also, like Gilgamesh from Final Fantasy V gets fooled easily when it comes to trying to obtain certain swords (Yes, swords). This isn't written in the story but the main character might give Scizor a chicken sword and say that it's the ultimate sword, making Scizor believe it, main character gives it to him and scizor attacks them with it only for it to deal no damage to the team then screaming the famous line of Gilgamesh "I'VE BEEN HAD! THIS IS BY FAR THE STRONGEST OF SWORDS!"