It is a fair question to ask.
I agree. I will note that it isn't the same question. One is about how good the candidate is at the job, and one is about basic ability to function.
To use a sports analogy, it's the difference between a guy who doesn't throw well being a QB, and a guy without arms being a QB.
I'll note that Fetterman wasn't the sharpest tool in the shed even before his stroke.
ETA: I just remembered that Walker had mental health issues. Is that what you were referring to?