If it were just a hat or a tshirt, I don't see an issue but, yeah, fully deck out in the merch of another company or an indie is pretty weird.I'm not saying that Vtuber fans should go the way of Sport fans where they have fights with people who support another team, but man, you're there for Hololive/Niji/Phase/whichever company you're paying for
Why would you willingly go to an event and wear merch of ANOTHER company
The only exception would be if someone had Ui merch.