Indeed there isnt. Its just a very unpopular opinion, since a looooooot of people love them theyr Mewtwo, for good reasons too (though his Melee incarnatio was admittedly lacking, a lot). Heck even Nintendo thinks so if they go as far as making the only piece of DLC him (as far as we know), and hype him up at the end of the Extravaganza (not to mention Gamefreak seems to like him a lot as well as they used him as a part of theyr hype mashine for X and Y). He was kinda a big deal once announced as DLC so having one person call him outright garbage will of course meet few wierd looks, but were all pals here so it wont amount to anything melicious and only friendly poking at best.
Please understand.