Yeah, flamingokid-x, you could be right! I had to borrow the DVD from the library again to check this out. It sure does look like Jack Nicholson!---especially if you compare this with how he looks in a production photo from 1963's The Raven.
In the T.A.M.I. Show, Nicholson's not really looking at Mick at the time, is he? But he's still smiling! Could it be that he's smiling because he's looking at the go-go dancers? Those go-go dancers included Teri Garr and Toni Basil. I don't know if he, Garr, and Basil knew each other before the T.A.M.I. Show, or because of it or what, but all three would go on to do Head (1968) (Nicholson produced, wrote, and had an uncredited appearance in this), and Basil and Nicholson would also go on to do Five Easy Pieces (1970), and Easy Rider (1969). Anyway, I just thought I'd throw that out there...
Now you've got me wondering who else can be spotted in the audience!
By the way, here's the link to that T.A.M.I. Show trailer that has John Landis commenting:
sorry folks, he looks NOTHING like Jack Nicholson...nothing!!