Typical Mario Bros. composer Koji Kondo wasn't involved with these songs, and it shows because of how vastly different in nature these two tracks are. The first, "Birabuto Kingdom" is bouncy and happy, stopping and starting for some chimes that add a degree of whimsy. It's the song from the first level and while not quite as iconic as the original Mario Bros. theme, it's captivating in its own way.
The second piece is Muda Kingdom and while not quite as bouncy, is incredibly pleasant and smooth. The tones aren't as high or shrill, they're milder and more relaxing. Beneath the main melody is a kind of bass track that moves up and down to create a more dynamic tone to compliment the main part.
Both songs have a relatively short play time until the being to loop, this being largely because of the limitations of memory within the Game Boy cartridge; you couldn't make long songs with limited space. Fortunately, Tanaka knew this and crafted melodies that you don't mind hearing on repeat because they have enough of a hook and pleasant nature to where they don't grow old.
Sometimes you can do a lot with a little when you need to, Tanaka showed that on the Super Mario Land soundtrack, using the Game Boy's chip to create tight melodies with a bright, shining, unique identity.