I haven't heard of any problems with the go!/Mesa pilots getting denied jumpseats, but I wouldn't really be surprised (given the lawsuits going on between a couple Hawaiian carriers and go!)
As far as your asking about hatred and animosity towards Mesa pilots, I personally feel like it exists, but have yet to experience it when it comes to jumpseating. Having commuted for two years, I've never had a problem, and can say that I've been treated with the utmost professionalism by crews that I interact with.
I commute primarily on United and American (mainline) and have been treated exceptionally by both airlines. It does help how one presents themselves to the crew, however. I make it a point to introduce myself to the entire crew (including flight attendants), have ALL forms of ID out to present, and with an extended handshake request the privelege of riding aboard their aircraft (either in the front, or the back). Obviously, I treat the gate agent with the same professionalism and courtesy...although that is rarely reciprocated by them
I have heard that Freedom pilots have been getting denied, which is really unfortunate (if one knows the whole story). I'm all for the original Freedom "A" guys getting denied (or at least a stern talking to) but to deny the current Freedom pilots makes no sense. Hopefully people will understand that Freedom is simply a certificate now (and since the ERJ wasn't a part of the "evil" Freedom of old, I'm not even sure if an "A" lister is flying Freedom...which was all CRJ-related). The damage was done with the "A" listers...and hopefully that list of names will continue to circulate.