Hey, welcome. Big piece here, lots of things. A lot of the issues relate to core game graphic needs. In games you need very clear readability of things important to the gameplay.
You need to be able to spot places you can and cannot go at a glimpse. Same with the player and interactables like the seeds and spring. They really need to stand out from the background. This is what people usually mean when they say things need to "pop".
Your level is very flat and difficult to read mainly due to contrast. The things that stick out to my eyes immediately are the dark trees. They are the darkest thing in the whole picture, and are sitting against the brightest thing, the sky. In other words, your highest point of contrast is in the background!

This is a quick and messy edit, but reducing the contrast in the background and adding more blue (cooling the temperature), and upping the contrast in the foreground and adding yellow (warming) helps to improve the readability. The eyes are forced to focus on the actual playable part of the game. The trees still stand out, though. You need to simplify the background:

If its meant to look cartoony, that means simplifying detail and exaggerating proportions (usually).

This is perhaps taking it a bit extreme, but the concepts are there: low contrast, simple shapes, lots of soft edges. Keep it impressionistic, no need for detail in the background, it will only really distract the eye.
The other thing about the piece is a complete lack of
shadow! Shadow is your #1 tool for describing form. Check out some literature on shading.
Also I suggest having a look at games like Mario, Zelda, etc (theres tonnes) that use a very cartoony style, but read well:
http://www.playright.dk/screens/superfrog_ami_01.pnghttp://www.onehertz.com/portfolio/wordpress/smw/schemes/http://lh3.ggpht.com/_AUMtcbXDbUI/SsGvJ-Fe97I/AAAAAAAAAuQ/vpbz9lc1VBU/s400/Picture%20191.pnghttp://img.gamespot.com/gamespot/images/2002/gba/yoshi/yoshi_screen001.jpgAs for karate kick... I don't think it really suits the character. He has tiny legs. Something more like pecking would suit.