I too wondered this as I started to MTB this year. So far, I have to say the MTB crowd is substantially more open and friendly.
I go a decent amount of group rides with the local club in Indy. There is a great number of roadies that are very nice and friendly on the weekend rides. They are freds I am sure. Then there are always a good number of the better riders that come off like deeks. Giving mean looks to everyone, obviously evaluating equipment, dress, etc. The weeknight training rides have many more of the deekhead types.

But at the local MTB trail (indy has few) which can be quite crowded - EVERYONE seems to say hey. People are friendly when they come up behind and need to pass. People are friendly as they pass. People are friendly as they are passed.
Now, there are signs at the start that are posted by the local MTB club that ask in no uncertain terms that riders be nice to others, to the trail, etc. Making it clear that being nice and maintaining the trail is the best way to keep the trail and welcome new riders.