Supergirl is best approached as good, campy fun. It feels like everyone knows exactly what kind of movie it is, except poor Helen Slater. She was served much better by later roles.
And given the WB/Universal SDS joint venture, it's largely a moot point. No Time To Die, for example, carried both MGM and Universal logos on the spine, IIRC. Most MGM titles are distributed by WB and carry both WB and MGM logos on the spine. Where it gets weird is in Canada, where MGM struck...
I have the digital 4K of Superman: The Movie; since SDS is no longer issuing digital codes with their titles in Canada, I'll be in for the first two, and that's it.