eh...
It was a little disappointing. There was no reason for him being in the void, no introduction, and no plot in general (well, not until the last three seconds of the flash, anyways). Also, as limited as sprites may be, that doesn't mean you need to over-use the recognizable ones. Every time Sonic got hit, you used the "death" sprite, and the result was was noticeably redundent.
On a tangent, whether your original idea was to place Mario in a void or you just couldn't think of a background, you might want to rethink it. To the casual observer, placing Mario in a near-blank environment looks more lazy than plot oriented.
It is good from a beginner's standpoint, but you have a lot of work to do before this flash does more than just take up space.