You can make and edit midis in Anvil studio.
MP3 is a digital audio format for recorded audio and MIDI is digital musical instruments (essentially a set of instructions for instrument samples). Converting to MIDI from typical music formats like MP3 rarely translates well, if at all because MP3 and MIDI are fundamentally different types of music files.
Likely the MP3 you converted only has 1 track and sounds like a garbled mess. A song made in the midi format will look like this in Anvil Studio.
Your best option will be to try and find the song as a midi. Here’s a link to the music repository on this website, it don’t think it’s been updated in a while though. Music Repository v3
There is also a music chat on the FEU discord, you’ll find more knowledgeable people on this subject than me over there. Link to the Discord is on the getting started post Getting Started
This post might be helpful if you’re trying to rip music from another game A Comprehensive Guide to Ripping Music for Use In GBA FE