She does have potential
South Park
Everything related to the animated television show South Park! (Images, videos, quotes, news, articles, thoughts, trivia, etc.)
She is supporting character not main so makes sense to me, but she has plenty more development. Pre-school episode has Shelley protecting her brother
Turd
Shut up, turd!
Ike was also undevelopted, but over time some major plotlines flushed out his story, so now he is just adopted.
I think there just isn' much to do with her character since we don't even know her friends. Many characters got created as background characters yet Shelly still had plenty of one liners in early seasons. The thing is that several of those background-like characters eventually got development and relevance, like Randy or Butters because they had ideas for new characters but didn't feel like creating new designs. But it couldn't be the same case for all of them.
Considering how good her song "Turds" is, and how violent her version of "I Saw Three Ships (Some Sailing In)" proved, she should get more musical turns.
She somewhat cares for Stan as seen in that Season 8 episode where Stan was scarred of a former bully