If you are writing in the style of a japanese light novel, with a japanese protagonist, then it's fine even if the author is western. Actually, I'd be more pissed if I had to read shit like 'Elder sister' over Onee-chan. Much like Korea, Japanese honorifics are necessary to the immersion, and as someone who translated light novels, I simply find the sheer lack of proper honorifics in English to be unsuitable most of the time.
Conveying the subtle nuance of honorifics like -sama and -chan, being able to understand why it's important that gender-neutral words like yatsu to refer to someone instead of He/She/Taco allows for misunderstandings to properly occur, as that is a huge thing in japanese culture, especially when you can omit topics and lead to problems when eavesdropping on something you don't know the entirety of.
Also, you simply can't translate the nuance of atashi/boku/ore with ease into English without destroying countless setups, especially when being used by an opposing gender. Just go look at google translate when trying to read ahead. How many females get labled as 'Mr.' because of -san just being the arbitrary blanket translation?
Do you wanna read about Mr. Anna?
Well knowing the people here, and their love for gender bender, probably so.
As for matters of other cultures in Isekai conversing in japanese with honorifics despite it not being cultural for them... isn't most of the time there some magic in play that allows the main character to communicate with them? I can only imagine that god's version of live google-translate is a hell of a lot better than what's available with the technology we have now.
I don't say any of this lightly or without weight. I have about 34 years of experience in japanese with a JLPT2 cert, and a minor in English which I obtained without even needing to bring out the tentacles in order to seduce my professor to back this up.
But just as there are weebs who want that immersion, there are of course English speaking/reading purists who are the same people that will never do a single risky or adventurous thing in their whole life and somehow find their sterile world to be acceptable, and that's just how it is.