Is Moxxi's Good Touch supposed to vibrate?

Yes. The constant vibration is the primary special effect of that particular gun (though it also has the heal-on-damage all Moxxi-branded weapons get). And no, there is no way to turn it off completely other than simply turning off vibration entirely.

The Wikia wiki on Borderlands has an entire page about the special effects of various weapons, if you ever need a reference.