I've seen some great UIs that are cobbled together from bootstrap + several extensions with a bunch of monkey-patched CSS and other jQuery extensions... the code is a hodge-podge mess that I didn't enjoy working with, but the UI itself was quite pretty/useful.

I've seen the opposite as well. One does not beget the other. On my current project, I'm often doing a followup pass on the UI/UX for this project.