Deobfuscator3000 but with yarn/tiny support
Decompiles minecraft mods and converts to named/mojang(legacy) namespace
Download deobfuscator4000 or build them yourself, run it, select paths in the UI, press Decompile button and enjoy src of mod without mappings!
For forge 1.12.2 you can download it from Deobfuscator3000's repo
For fabric you can download it from https://maven.fabricmc.net/net/fabricmc/yarn/VER/yarn-VER-tiny.gz where VER is version of mappings. You are able to find it on fabric's website
SimplyProgrammer for the original Deobfuscator3000