Lisa: I don't think the real Radioactive Man wears a plastic smock with a picture of himself on it.
Milhouse: He would on Halloween!
This is a reference to the c Halloween costumes from the 1950s to the 1980s manufactured by companies like Ben Cooper Inc. and Collegeville, usually consisting of a simple plastic smock of body suit and a plastic face mask. Usually the smock would have a picture of the character of the costume right on it; Milhouse's Radioactive Man costume consists of a similar plastic mask and a smock with a picture of Radioactive Man saying "Trick-or-treat!" on it.