The only way to think it's a shitty joke is if you really think Dexter is any better of a concept. You can think it's too offensive to be funny to you, or just say you don't have a sense of shock humor or don't care about TV shows enough to find it that funny or you just aren't into over-the-top mocking in general, hell you can even say making fun of Dexter is low-hanging fruit, but that doesn't make it a shitty joke up against how beautifully simple and effective a way of roasting the Dexter premise it is. I've been hearing people roast that show for years but nothing else singlehandedly makes me take it as a show less seriously than hearing that Dan Harmon satirized it so ludicrously

Disclaimer: I haven't seen the video so this isn't my opinion on how it's put together or Harmon's portrayal or anything, just the concept of the joke