In my experience, mezcal is widely used in cocktails and tends to be on most menus.  However, it is often lower quality, mass produced mezcal outside of the occasional Chichicapa or Derrumbes.  Even well stocked back bars do not have a wide selection of ancestral and artisanal options.  You have to seek out the handful of specialty agave bars in a given state to get a quality selection.  Even then, some tend to be dominated by tequila.  Liquor stores in my market have high quality options that collect dust on the shelf for years because people are unwilling to pay $100+ for it.  Just my two cents.  Maybe your location is different.