nice and clean ... i like it ...

I'll try, one day :)
I'm learning flash now, and it seems pretty convenient for a game like this :P

make this a game damn!

Glad to see that something good came out of this epidemic *cough

omg no robots O_O
gotta fix that O_o

Nice mockup but you forgot robots.

Very nice! The clouds on top right pic are great, and I like the sweet little trees in the winter level, as well as the tiny skeletons. I'm not sure about the gray outlines on the bunny, at least not together with the black suit and dark BG on the bottom right pic. Mostly it's a good idea, IMO, to make the main char pop from the BG with increased contrast, outlining or differences in the palette, although it might work good anyways when the character has been animated.

Overall, very nice mockups!

Cool. All I could say that may help is some more background art. Background art really pulls you into the game. Maybe a floating island with a water fall coming off of it, more clouds, or a hot desert sun in that pyramid picture. Great job otherwise!

Really great mockups

Awesome, I love the Skeleton boss and the clouds! 

Looks adorable.