Very enjoyable. Reminiscent of a familiar classic to which many people will readily identify, but not imposing on that title. Well done. However, if someone hasn't, someone should check public domain of some of the sounds. A few are blatantly reused and I would hate to see something so likable see infringement issues.
Otherwise, only a few brief critiques. The second flower (fire-power) in World 1-1 is too high to obtain. And in the straight-drop of the double row of coins in World 1-2, one of the coins is on a brick, thus impossible to get (unless there's an unseen trick). I'll keep playing and if anything else is relevant enough to raise concern, I'll post more.
I like how the character looks - similar to a bad guy from one of the old games, how it appears that the mushroom hits him like steroids, and how the flower makes him look like a bad-ass with his hat cocked. Very well-created.