What kind of phone or tablet? It works great on my iPhone / iPad, better than most mobile sites. I know that the Android browsers zoom differently than Safari on iOS, maybe that's the difference?
Comment pages look okay on your iphone when there is six or seven levels of replies? I switch to iheckernews.com if I want to read a long comment thread on the iphone.
You can't vote (you have to zoom 3x or you'd accidentally down-vote a poor guy), and the front page is not readable in portrait mode, and only barely legible in landscape mode. Nested comments (more than 4 levels deep) are usually unreadable.
One rather handy feature of Chrome on Android is showing a small, zoomed window of where you click when there are a few links together; allowing you to select the correct link. I also assume this feature is present on the iOS version of Chrome.
Google "Responsive Web Design." you shouldn't have to pinch your zoom your way into every website. HN has a poor mobile experience.