I suppose. I don't tend to use endearments myself in any language, so I probably can't judge. (And my parents used nicknames derived from their first names for each other, and stupid pet names for only their kids, e.g. in they called me "Tüffelchen" of all things, which is "little potato" from the Low German "Tüffel" meaning both potato and "inept, clumsy person", so I could probably claim emotional scarring just from that, though admittedly "Mäuschen" also featured, which is nicer.) But in any case if authors want to go that route they need to make sure the words are right.

Bu if it's any consolation, iirc as a long term historical trend case endings are on the decline in German, in that several are already look the same and fall together. And just look at the genetive which is practically extinct already in spoken language, so it will only be a few more centuries at most until it's like English. Which might be why the word order isn't as flexible as in Latin.