They're clear and consistent and a lot better than half a dozen different lines all just being called "Overground". They've explained the meanings behind the names in several pages on their website at https://tfl.gov.uk/modes/london-overground/overground-line-naming.Who the heck is Olivia Dean? There's one false assumption they've made, straight away - that everyone will actually get their silly joke.
Confusing and daft. Put it back. And while we're at it TfL can get rid of the confusing meaningless alternatives for clear and obvious names like "North London Line" etc.