That new MewThree(or whatever it will be called) just shows the huge gap between the designs of the older pokemon and the new ones. Though I will say it's possible the new one is a more accurate and successful clone of Mew, therefor it kind of justifies the "cute" and soft, round design. Otherwise you can see the big difference between the two:

http://static.giantbomb.com/uploads/original/9/95666/1890582-mewtwo_strikes_back_film_artwork_return_kanto_gen_i.png.jpg (Unsupported image host)

http://images.nintendolife.com/news/2013/04/new_mewtwo_form_revealed_for_pokemon_x_and_y/attachment/0/large.jpg (Unsupported image host)

Mewtwo had so many sharp edges and wasn't afraid of an angle or two in the lines. Pointy joints, muscular-like chest and pointy eyes with a bit of anger behind them. The new one is round, round... and more round.