It's a port of the original Super Mario Bros from the NES to the Game Boy Color. The physics feel very nice, and I like how the sprites are adapted.
The port also extends the game. After the 8 original worlds, Quest B adds 8 more with new enemies, mechanics, and even bosses! I had so much fun playing it.
Also, the game is FOSS: https://github.com/Mico27/SuperMarioBrosMini
Interesting, then I'd say calling it a port is doing it a disservice if it's a full remake!
Ah, your are right!
The author calls it "demake". I guess it's fair due to GBC decreased resolution... But it adds so many features, I would call it "remake".