They use NTSC-J, which is slightly different. I remember it as being different enough to be incompatible with US NTSC, but it doesn't look like it.

https://en.wikipedia.org/wiki/NTSC#NTSC-J

However, Japan uses both 60hz and 50hz power, so I wonder what that does to frame rate.

Yep, Game Boy was a different medium. And Nintendo's artists didn't just take the Mario graphics assets from the NES and plop them on a Game Boy; that would have looked like shit for a number of reasons. Yet that is what people do when they take the 8-bit-era Mario sprite and render it with giant squares.

Graphics assets are no more a picture than a recording of an artist's brush strokes are. They can only be interpreted in the context of a medium, be it CRT scanlines vs. LCD squares or oil-on-canvas vs. ink-on-papyrus.