I'd prefer a bit more separation between up / downvote buttons.

I'd prefer the text to be a bit larger, although I accept that some people would hate larger text. Is this the kind of thing that HN could have a setting for? optional style sheets? (Because users can't set their own styles on mobile).

Stylish works great with Firefox on my phone. Doesn't help you if you're using another browser or stuck with a phone which doesn't let you choose your own browser (do those exist?), of course.