There almost certainly is cycling snobbery where some look down on others who they consider to be on inferior machines or not "with it" because they don't dress in slogan / brand emblazoned lycra, but I would guess the lack of a wave or greeting from other cyclists has just as much to do with different personality types, and even the mood of people.
I tend to acknowledge other cyclists whether roadies, MTBers, or people on shopping bikes, some respond, others don't. I have been bimbling along on my hybrid in my Ronhill trackies and baggy top several times when a club ride or similar group of fast roadies has passed and have been on the receiving end of a chorus of "hellos" and "good mornings", I was struggling up a hill a few months ago when a very pleasant lycra clad roadie on a dream machine pulled alongside me, slowed down and chatted with me for a couple of minutes.
On the other hand some cyclists don't acknowledge and just ignore me, so what! They might be a shy introvert type, have problems on their mind, be lacking in social skills, just plain ignorant or cycling snobs. Like every walk of life, it takes all sorts.