Played through the few first chapters of this over the weekend. The gameplay isn't half bad and I can see it getting better once I get more moves. But it is a struggle to get past the hideous art direction, not a single memorable or likable character and grade school level dialogue. Even by Saturday morning cartoon standards the writing is atrocious. I played about 3 hours total the last two days and this morning I couldn't tell you the name of any character besides Knack.
I agree with the bolded, but honestly - I really don't see it as a big deal, from recent memory - Ratchet & Clank, Crash Bandicoot and Jak & Daxter were all the same for me to varying degrees. The story, dialogue and characters (excluding the MC) are forgettable at best, it's just not something that really needs to be fleshed out in a platformer (not to say they can't - A Crack in Time for example was great), which is something the Mario titles are fully aware of. If there is to be a Knack 3, I hope they forgo trying to tell a story and just keep it simple.