I'm not sure about likeable, but maybe "relatable" to some as I recall Genndy explaining somewhere that much of Dee Dee's belittling behavior stems from his insecurities.
Genndy's was the more art inclined child to his brainy brother and felt increasingly unsettled as they approached their professions.
Dee Dee similarly was the only child with nothing to prove at first, and was even overjoyed when Dexter was born as she thought he'd be her compliant playmate, but when he begins to prove indifferent to her and capable in whatever he put his mind to the division starts.
Some might think this a stretch since she usually just causes chaos with no seeming reason and gleeful abandon, but there's a scene where Dexter ridicules her about being simpleminded and she suddenly goes on a rant about being prettier, taller and having more friends than him. So she's mostly acting out, she doesn't hate Dexter, just how Dexter makes her feel small.
There are other scenes where if something or someone is troubling him she's concerned, and other scenes where she even showers him with affection when he's nice to her, but the sibling rivalry resumes whenever the moment passes.