Mountain bikers care about appearance. Deeply. I'll never deflect the issue. Many an early weekend morning has been spent anxiously rethinking my riding gear.
But beyond the issue of fashion, there is the requirement for function. And in mountain biking, that means adequately robust kit to resist tearing when snagged by tree branches without sacrificing fabric breathability.
I don't love riding in lycra. It's a personal bias. There's no question that a premium lycra bib and jersey are terrifically comfortable and lightweight, but the aesthetic is beyond my sense of modesty. And the damage risk, if you crash and tear lycra, is very real. The moment it tears, it's ruined.