This shirt is Exhibit A in the argument that, yes, Cage knows exactly what's going on.
Guns N' Roses (at this point, Axl Rose and a rotating cast of stringers, including, coincidentally, a former member of The Replacements) posted this picture on Instagram after the group's June 7 show at the The Hard Rock Hotel & Casino in Las Vegas.
In case most of your synapses have fried, it's a picture of Andrew Dice Clay standing next to Nicolas Cage, who's wearing a cowboy hat, aviator sunglasses, a blue blazer, Mardi Gras beads, what appears to be a set of holsters, (possibly) fringed pants and – most crucially – a shirt with his own face own it, perhaps inspired by Macaulay Culkin's habit of doing so.
The cartoon on his shirt is commonly referred to as "You don't say?" It's a facial expression Cage makes in the film Vampire's Kiss, and it's used in online discussions to "express frustration or condescending incredulity at an incredibly obvious or oblivious statement."
Perhaps it's commentary?