Whoa, Mario even killed Darth Vader *points at bottom left corner*

Wow. Even nailed the wonky perspective. This is really impressive.

What piece of art is this a parody of?

That would be "Saint George Slaying the Dragon" There are a lot of medieval depictions of the scene, but the constants are almost always:
1) St. George on his horse, rearing up, and stabbing down with a lance or spear.
2) The dragon laying on the ground and being stabbed, defeated.

This isn't a direct off any one piece that I can see, but it definitely fits the bill.