Whew, I think if I were playing a game with that background, I'd get dizzy and have to stop.
Really, those are some bright looking patterns. What's the kind of look you're going for? It seems like you want something you'd see out of a Yoshi or Mario game. (I say that based on those hills). If you insist on those zig-zag stripes, then at least make them thicker/further apart/not so close. You should also add some depth to them, and change the lines to show that they're wrapping around something round. (Again, I'm just assuming you want something round, based on the hills).
This basket ball is a good example on the lines:
Notice how they bend? (Now, it's possible that they're meant to curve or something, but this should still show my point). The object is round and curvy, so the lines are too.
And like bengoshia said, you need to define a light source. You're at least showing depth by making the "hills" further back dark, (which isn't what you want to do, but works to show depth). When things are going into the sky, they start blending with with it's color. So, instead of making those hills darker, you should make them lighter and bluer. (Depending on how far back they are of course.)
Also, grass isn't flat.
Your grass shouldn't be super tall or anything, but you should show some stands or clumps of it. Doing that will make it look a lot more like grass.
I think you should also try to keep your sprites and background look, "correct" together. That lizard looks out of place because it's got earthy tones, and isn't too vibrant. (But I think you should tone down the vibrancy in the characters and background anyhow). In any case, no matter what graphical direction you go in, it should look like they're suppose to be there.
And I don't know what those balls at the top are suppose to be, but they'd look better sprited.
And you should keep your text consistent... Size could change I suppose, but I wouldn't change the color and outlines like you've done. Try to make it look similar so it doesn't look so slapped together. And you could make a new icon for the lives instead of just taking the character's head like that.
And what sprite sheet did you color exactly?