I almost agree with you, i think there could be people who can separate both things, tbh It's not the case for me. I see what you're saying, for example, when many people prefer metal bands where the musicians look like "real" metalheads. Many people dismiss metal musicians when they are short-haired/obese/etc or if they're not wearing "metal" clothes. Maybe this is more true in the local scenes, or at least that's what I've observed here in Mexico. Maybe it's not exactly what we are discussing, but going back to the subject, i think it's difficult (at least for me) to separate the impression I'm getting (because of the tweets) and my excitement about the upcoming SoA's album.
I suppose there could be individuals who can wholly separate things, but they are far from neurotypical and are probably severely cognitively impaired. I know musicians not looking as metal affects my experience, though. I'm going to enjoy a concert a little bit more if all the guys on stage look badass or otherwise fit my definition of 'cool' (which fluctuates by genre). It's absolutely shallow, but it's the animal in me (and all of us). At the same time, someone into nu metal or djent or whatever is popular now might be thrown off by the looks I think are cool ('these guys look like '90s throwbacks'). Likewise, if someone thinks Dream Theater looks best with bald Jordan and inhabitable beard JP, then they're not going to agree with me that they looked cooler in the '90s, and we're going to have differing ideal concert experiences.
Whatever your personal ideas of how a band should look (or do anything else), it's classic bias based on past experience and it's in all of us. It's not much of a detriment to society when it comes to something trivial like our perception of musicians and their output, but it's a bias which we're better people if we suppress when it comes to people who look a certain way in every day life... It's often a moral decision to suppress ancient survival instincts, but it's not possible to do it perfectly.
I don't think it's that black and white. I haven't been a fan of Mike Portnoy the person since the early 00s, yet I still have bought tons of CDs he has been a part of, and quite a few of them released since then would be on my list of all-time favorites. I think Eddie Van Halen is a piece of maggot shit, but I still enjoyed the heck out of the last album they released (and can still listen to VH and enjoy it without letting my dislike of EVH the person get in the way).
What's certain is that it does have
some sort of effect, but it's a fine gradient as to how much and the shade can be so light as to be undetectable--but it's always there. Do you think your VH listening experience might be just a little bit more pleasurable if you didn't occasionally find yourself thinking 'Eddie is/was a great writer/player--it's too bad he is also a pile of maggot shit'?