Hi! It seems there were some changes in 1.21 that make the mod no longer work (even when version bypassed in Fabric):
[Render thread/INFO]: fpsdisplay.mixins.json:MinecraftClientAccessor from mod fpsdisplay->@Accessor[FIELD_GETTER]::getCurrentFps()I should be static as its target is

Thank you!!

Just released an update for 1.21, enjoy!
https://modrinth.com/mod/fpsdisplay/version/4.2.0+1.21