If you count in the fact that any idiot might just hit you down with their car after mixing pedals or some kid run in front of you, it's just obvious to wear a helmet unless if you're an idiot.
Using a helmet is basically comparable to a seat belt. Zero effort really, just a habit you're supposed to learn from childhood and automatic ever since.
How would you call a person refusing to use seatbelt other than a idiot?
You should stop mountainbiking if you really care about risk avoidance so much, "unless you are stupid and an idiot". It's much riskier than casual cycling without a helmet.
Yes exiting your house is also a huge risk, and statistically staying home is even bigger risk. Better just lock yourself to a soft room in an institute or commit unalive. /s
You clearly seem unable to understand what a stupid risk is. You're ignorant to the point you'd take a risk with zero possibility for a positive outcome, just for your fragile ego. Just like the rednecks too cool to wear a seatbelt or a construction worker at hights without harness.
I'd call it fair if it was for the thrill, that's cool and we all have done and are doing stupid dangerous things for it. That's maybe not always smart, but it's what keeps us alive. That's what we're talking about when we think of calculated risks. If I hit this trail, there's a chance I'll hit a tree if I make a mistake. Or I'll grind that rock garden with my face. I'm well aware of those risks.
Stupidity is to imagine those risks don't exist or let your ego ignore them because "I'm so fucking tough" or "I don't want to mess my haircut and I'm not gonna fall anyways I'm not a kid"