I love a good comeback, but a comeback is only as strong as what it's coming back to. If someone just says, "you suck" there isn't much to go off of. You usually have to say something equally as stupid back (I usually go with "you too!") Then there are jabs that have enough meat on the bone for you to build off of and come up with something clever. That's the unintentional case with these shirts. "Can you walk the walk?" is a loaded question. It's no "you suck."
Serendipitously, this other guy's t-shirt has the perfect comeback. "You walk. I fly." These guys' shirts are like an 80s buddy cop duo. The shirt on the left is Sylvester Stallone and the one on the right is Kurt Russell. This is the fashion equivalent of 'Tango and Cash.'