This is apparently a fan-made spinoff of Super Mario World starring Marisa from the well-known Touhou series.
Here's a download link.
Having completed the game, I wouldn't recommend playing through all twenty stages just for the ending. It's very disappointing for all the work you have to put into it. The game's very easy except for a few frustrating spots in the later levels, but the gameplay gets repetitive quickly.
After the credits, Marisa (as an adult) walks across the screen. For about half a second, you can get a glimpse of a girl standing at the right edge, but Marisa immediately throws a mushroom at her. The girl looks shocked, the screen goes black, and you hear the "shrinking" sound effect. Afterward, you unlock the ability to play as a shrunken Alice, who is exactly the same as Marisa except that growth mushrooms are fatal to her, leaving her stuck at head proportion 2, and she can throw little Marisa dolls at enemies. She doesn't even have her own ending.
The AP artwork for Marisa is quite good, but you can spare yourself the effort of playing through the game by looking at the screenshots. Unless you feel like an hour or two of drudgery, don't bother playing this.