Depends. I’m a fan of dressing to make a statement. Either to reinforce your sound, or subvert it, whichever us more appropriate.
Right now I’m preferring semi-formal grey jeans, black shirt, black shoes for my current tech hardcore band where everyone else in bands and audiences are mostly in band shirts, vest tops and snapback hats.
Adds a bit of class and makes you stand out from the audience.
I also think it’s important for the band to co-ordinate appearance so you look like a unit on- and off-stage. In my band we all wear plain black tops and dark jeans, and the vocalist is the only one wearing colours, which again makes her stand out and focuses attention on the frontperson.
Whatever the attire, it always should be deliberate and have some thought put into it.
It’s like dressing up for work; it puts you in the zone and makes you feel a bit more engaged and makes it easier to perform your role.
Casual jeans and a band t-shirt looks a bit lazy; maybe just me but I’m always disappointed when a band looks like they just rolled out of bed and didn’t think to put any effort into how they present themselves.