Think it'll be hard to define in any sufficiently specific manner legally because UI/UX/source/functionality aren't entirely separate, but ethically I reckon it comes down to what one means by copy UI.

e.g. If you're creating an uptime dashboard...they all kinda look the same anyway and there aren't that many ways to do it so that seems OK. If it's copying an comprehensive UI with layout and flow between the various pages etc then you're getting a bit closer to theft.

That is why we have judges, to make judgement calls. Its probably going to come down to how closely you are copying.