The first movie set the bar pretty high and unfortunately this movie while ambitious failed to hit the mark its predecessor established.
While the animators did an exceptional job making a film look great in CG, nothing was done to really fix any of the story line flaws the first movie had.
There's also a LOT of padding in this film. Almost 30 minutes worth of unnecessarily stretched out scenes that drag on forever. They basically made a 70 minute long movie pad out into a nearly hundred minute long film. Like there are only 2 new scenes and they are quite forgettable. No expansion on Mewtwo's origin. No added lore. Just more action and flash.
I'm sorry to say but there's nothing about this movie that makes it all that superior to the first movie save for the sleak new animation. You are better off just watching the original 2D movie.