The User Experience designer's corollary to technical debt is experience rot. Over time, a poorly designed user experience gets worse and worse, especially if there isn't concentrated effort to improve it.
Experience rot happens when features are launched with half-assed user interfaces and then abandoned. It’s created by launching feature after feature until they pile up like crappy plastic merchandise in a dollar store.
Like technical debt, you need to refactor the experience every time you're in the code. Every user story should create a visual change.