Princess Diana has an anime yes. It's very short & only aired a single time. Is it good? It's more of a historical oddity than something you'd really watch on it's own, but the fact it even exists in the first place is worth talking about. This is where I'm just going to post supplemental info now. The company that made this and most of the people who worked on it aren't really well know, nor have they worked on things most people have heard of. It was done very quickly & only meant to be seen once to coincide with her visiting japan in 1986. I doubt they even thought anyone in the current year would be talking about it. That said, I thought it was worth mentioning at least once. Maybe someone will subtitle it?
I stole this from some guy on telegram. It's probably the best avatar I've ever seen. Watch season 1 episode 18 of Re:Zero if you don't understand why.
Code Green: The dubbed version is preferable, as the quality of the dub is superior to the Japanese language version, or different languages are portrayed in-universe in such a way that it would make less sense if the characters usually spoke Japanese (e.g., Black Lagoon). Watch it with subtitles only if you're a purist.
Code Blue: Both the dubbed and subtitled versions are passable, and choosing one or the other depends on one's personal tastes. Most dubbed anime fall into this category.
Code Yellow: The subtitled version is preferable, as the quality of the dub is markedly subpar, or different languages are portrayed in-universe in such a way that it would make less sense if the characters usually spoke the language into which it's dubbed (e.g., the English dub of Azumanga Daioh). Watch it dubbed only if you really can't stand subtitles.
Code Red: The dubbed version is completely out of the question, as it's been corrupted by fucking social justice warriors (e.g., the English dubs of Prison School and Miss Kobayashi's Dragon Maid).