so I've been wondering, is it just a very popular headcanon that mutant bloods only live a short time, or is that stated in canon somewhere and I missed it? thanks if you can answer!

It’s canon.  Troll lifespans directly correlate with their blood type.  The highbloods live longer, the low bloods live shorter and since the mutant blood is even lower than the lowest lowblood, they have the shortest lifespan.

~Mod T