Maybe, but that hasn’t been my experience. From Canada to Germany to Japan, I felt that people were less competitive and more welcoming. E. g. from the very beginning I was “taught” to wait at regular intervals for others on climbs, which made it much easier for people of different ability levels to ride together. Fawning over someone’s nice bike was also never a problem if they had knobby tires. Whereas that caused problems with roadies every once in a while (my brother and I were in “civilian” clothing). Mountain bikers would also more reliably greet back whereas some of the roadies here seem confused when I raise my hand or give a nod.
Perhaps if you find competitive groups of mountain bikers that changes. I can imagine it does when you want to go for a fast ride and you are stuck with someone much slower than you. But usually that’s fine.