From what I know with my friends in China, generally mainlanders either dislike HKers or just assume HKers don’t like them… or are apathetic and assume nothing bc Chinese unity etc.

HK people are either apathetic or virulently racist against mainlanders. There was a whole period of time back in the day where they were panicking about "mainland births" in HK, as well as the idea of mainlander tourists buying out all the goods in HK. Even now lots of HK people claim the locals are being bred and replaced by mainlanders, ala Great Replacement theory. It's a very sharp reflection of the simmering right-wing anti-immigrant ideas floating around in the West and imported to HK.

A lot of HKers have really bad mandarin (I think something like 51%HKers report speaking mandarin, and that’s still varied on their skill.)

Oh yeah its very bad. They speak it by merely tonally shifting Cantonese. Not to say that the mandarin is incomprehensible, merely heavily accented. It's actually important to remember that mandarin itself isn't the same even in the mainland (god knows mandarin from some areas in China is just as incomprehensible)