Hey, I'm new to this game and was wondering if there is a reason the mGBA emulator is not allowed.

The only reason would be lack of data, as well as lack of demand. Personally, I don't recall ever seeing an mGBA submission for any GB game I moderate.
That said, I would expect it to perform well and if you have a case to make for it, I'll be happy to consider your input.

Well I got a case for it. I'm on Manjaro linux, and neither gambatte (taken offline by the creator last year) nor bgb (windows only) are real options. It's stopped me from revisiting SML1/2 recently.

Gambatte-speedrun is still available, but can't get it to work. mGBA on the other hand works without any hassle, just compile and run.

I'll have a go with it to check that it's lag and bug accurate somewhere this week.