Spongebob Gets Ripped

Listen up, smoothbrain. The joke here is a wordplay on "get ripped." Most people think about getting muscular (like going to the gym), but Spongebob is a sea sponge. Sponges can literally be torn or "ripped" apart. The guy making the joke is comparing his lack of muscles to Spongebob's literal state, saying he can't even get ripped in the literal sense. You probably thought he meant Spongebob works out, didn't you?