It's almost always electric. Probably because there's license to be showier, all Marty-McFly-on-stage-with-Marvin-Berry-a
I've done that. In Grade 12 English we got talking about love stories and love songs and how on-the-nose obvious they can be, and without warning even to myself I mimed playing a guitar (mimed badly, I'll admit, but I have almost no experience on guitar) and improvised "My love, I love to give love, and love you, my love..." I didn't know it at the time, because that was 1991 or 1992 and Adam Sandler wasn't yet well-known, but it was kind of an Adam-Sandler-on-guitar kind of moment. Which was and can be kind of cute. But Air Guitar ain't usually cute.
P.S. Now I can totally picture s00j doing Acoustic Air Guitar. And doing it well and showily.