Roadies do have a disdainful term for mountain bikers. They call them "mountain bikers".

Mountain biking has Freds. They are known as "retro-grouches" or "hippies", and ride rigid multi-speed bikes. Note that riding a singlespeed rigid mtn bike will get you referred to as "nutjob", "goof" or "weirdo" but not as a retro-grouch or hippy. Sporting lycra sans baggy shorts means you are a "racer". Not saying hi to other trail users makes you a "******bag". Riding around the trailhead on a brand new bike [that is so new it still smells like the cardboard it was shipped in] while wearing brand new bike gear, but with your helmet clipped to the handlebars might get you labelled as a "new guy/girl". Riding the same bike around your local mall parking lot will get you labelled as a "poser".

These terms can be combined:
"Hey man! Did you see the poser? He looks like a racer, but I bet his ride has never seen dirt."
"I'm reading your mail dude. I hope that new girl remembers to put her helmet on. I thought she was a weirdo, but then I saw her talking to that ****ing hippy, and then..."
"Oh ****, here comes the hippy!"
"Quick! Hide your stash or he'll... too late!"
"Hey brah. Can I mooch on your stash, brah?"
"**** off hippy!"
"Whoa brah, what's all the negativity for? You're like, seriously making a dent in my soul, brah..."
"Dude, we just came here to ride. If you break into our car while we're riding, I'm gonna dent something for sure. Now, get back on your Miyata and get the **** away from us."
"Wow, brah. There is so much harsh energy here, I don't want to contaminate my aura." <rides off on 80's era Ridgerunner>
"Man, that was close. Let's hit it."
<roadie rides by to go do hill repeats on access road> "Goddamn mountain bikers!"
"Huh?"
"******bag."