MP3 downloads for audio files is highly requested, but bring some difficulties for our developers.
First, and most importantly, YouTube does not provide MP3 files which could be directly downloaded.
Therefore the only thing we could do on NewPipe's side is to convert/transcode M4A or WebM streams.
The reason why this features has not been implemented yet, is quite simple:
This process requires an external library like FFmpeg which would significantly increase NewPipe's size.
Nevertheless our team is working on a solution to make this features as soon as possible available to you. The plan is to move the conversion to an external and thus optional app called Video Transcoder which you can download from F-Droid too.
Help and contributions are always welcome. If you want to take a look at this issue, please read this page on GitHub.