Assuredly, if Fetterman could counterbalance his dress code with his actual ability to process, analyze, implement and conduct policies, the fact he his dressed like a bum would be the least of anybody's concern.
The problem is: it is not the case.
Cognition wise, before his election, he was reportedly "not all there".
Then came the stroke.
And since then it has been rather clear he still was suffering from long lasting side effects.
Now, that's not his fault and it is extremely unfortunate, but it does have clear effects on his ability to fulfill his duty. As he said himself: he is, at times, unable to process information, express himself, etc...
The dress-code is merely "icing on the cake", though I have no clue as to why he dresses like that.