Functional? Nah. I couldn't hit update. o.o Not functional on mobile, not for me anyway.
It's been a problem since day one for mobile users. I've had two phones since joining these forums (iPhone 4 and iPhone 7 Plus)
Both of which--same as OP--have been completely unable to update signatures, though avatars have been no problem. Actually came across this problem again two days ago, and it really can get annoying at times.