Alas, there aren't any diagonals on that tileset (Orange Grass). I'll look through and see if there's anything similar (I chose it for the tree models) but if I can't find anything better, I'm somewhat happy with it at the moment. (
Edit:Maybe a change to 'Red Forest of Beauty')
One thing I'm having trouble with is a very small part of that screen - I've added a small series of jumps to blocks to reach the first ledge, but they're just floating blocks - nothing holding them up (see below). I might scrap that idea and put the climb powerup before the hill.
Update: I've redesigned the mountain area (and backgrounds preceding it) based on LPChip's advice to smooth the backgrounds out. It looks a lot better, in my opinion. Thanks for the tip, LPChip! I've also decided to place the climb powerup under the tree and scrap the 'jump from floating blocks' idea.