Im using croptopia in my modpack. I would be really happy if there were quality items for this mod.

As long as the mod is using the proper tags it should already work
I tested it with some items in 1.21 and they were applicable to quality (incl. the drops from the trees)

Would it be possible to investigate the issue further? I'm using latest Croptopia and Quality Food mods on 1.20.1 version and got zero tiered seeds from bushes or by manually growing them with bone meal.

Trees indeed give a tiered fruits, but their quality also lost when making saplings from them.