He may improve, but it's getting clear that Fetterman has a lot of disabilities -- he went functionally deaf, that is maybe the worst.
I don't think it matters in terms of the work once they get to the Senate --- they have huge and competent staffs to manage everything and tell them how to vote if they need that; probably the party whip or leader tells the staff how they should vote. Look at Feinstein: quite unable to manage now, but she's still there, run by her staff.
These senators are an industry, a company hiring lots of people, so no one can let them quit.