Plenty of games are being written in C for the Game Boy on a regular basis.

Just a few polished examples of that: https://tangramgames.itch.io/tobu-tobu-girl-deluxe

https://user0x7f.itch.io/black-castle

https://binji.itch.io/porklikegb

https://pocketpixel.design/super-jetpak-dx-game-boy-rom.html

https://aiguanachein.itch.io/powa

https://thegreatgallus.itch.io/cavern-mvm-9

https://user0x7f.itch.io/gunship

https://www.infinitygbc.com/

ASM will tend to be more efficient, especially if written by someone with experience and skill, but that isn't enough to prevent other tools from being used successfully. It's described in much more detail in the linked article.