No, mGBA does not require a BIOS file to run GBA games. The emulator has its own built-in BIOS implementation that emulates the necessary functions of the GBA hardware. This means that you can simply download and run your GBA game ROMs in mGBA without needing to separately download a BIOS file. However, if you do have a GBA BIOS file, you can still use it with mGBA if you prefer.