From what I've shown to friends of mine (in the U.S) it's either one of the following:
1. Language Barrier ("I can't listen to it w/o understanding the lyrics" or "The language sounds "weird") [Totally understandable BTW]
2. Any guy who wears makeup/cross dresses/etc. is automatically gay (More specific to VK)
3. Your interest in Japanese pop culture was supposed to be short-lived fad from your teenage years ("You're still listening to anime music?"/ ASSIMILATE PLZ)
4. It's simply not my taste [Again, totally understandable]
...and something actually interesting to talk about....
5. The melodies are "cheesy".
Idk if this has been mentioned before, but whether it's the preconceived notions they get from watching idol groups or anime songs; "cheesy" gets brought up a lot.
Since I'm not really versed in Music Theory, I can't explain this very well... This first comment from "nonotan" does though (to some extent):
https://www.reddit.com/r/musictheory/comments/2e5v76/what_makes_some_japanese_music_sound_japanese/
( Also reaffirmed my love of DeBussy ( ͡° ͜ʖ ͡°) )
Now, w/ idol groups and certain J-pop artists (who cater to a certain demographic) it makes sense for them to be "cheesy" (your "Pretty Cure" openings will never be edgy =P).
Cheesiness can also be applicable visually, so many can find a lot of VK MVs to be really cheesy (additionally dependent on taste).
However, I've seen this comment with less flashy, run-of-the-mill J-artists and more importantly, instrumentals/OSTs.
I can't really delve into more since my knowledge is limited w/ this topic, but I just found #5 to be an interesting take on all of this.
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Also
6. YO JAPANESE MUSIC R SHIT CUZ OF PEARL HARBOR & THEY GOT FUCKED UP BY THE NUKES.
(Very, very, uncommon lol. Though that Women's World Cup (soccer/futbol/football) win caused a shit-ton of comments like that xD. People are strange)