Okay, first of all I like really like the title screen. The difficulty selection is okay too, but why is there no load function? At least I didn't find one. Then the basic stuff; you should replace some sprites on the stage selection screen and change the music (you did that after the first two screens, but yeah, maybe you'll change it anyway later on). Then, please slow down with the invisible blocks, everyone hates them.
The game itself is okay so far, there are some good jumps, but you also have really boring sections in it. Also the game design is rather confusing; I did not yet understand what you have to do with Mario.
Please pay more attention to details, because from what I've seen, you could make a decent fangame. And, yeah, give more information about your fangame in the OP, and attach some screenshots. People on this forums are very critical about everything that gets posted (sometimes for a reason).