The guilty pleasure anime category encompasses the shows you don't want anyone to know you're into. For some people, these are fan service anime - these series can be a ton of fun, but viewers might feel weird about them, either for moral reasons or simply because they'd be embarrassed to have someone walk in on them while they're watching. For others, a guilty pleasure show might be super cute in a way that doesn't fit their image.
There's no reason an adult can't enjoy Chi's Sweet Home, but it may be hard to explain to a non-fan why they're watching a show aimed at small children. A handful of shows just aren't well-written or are only good in parts. If someone is a literature major who regularly reads the classics, they might struggle to articulate why they're so passionate about Yu-Gi-Oh! Regardless of individual taste, there are a ton of popular shows people wouldn't necessarily boast about watching.