It surprises me. The upvotes and downvotes are gif images, so all you're seeing is a blown up gif. I'm just in the process of replacing similar triangular gifs with near identical looking Unicode characters precisely so that it will look better on mobile devices that make more use of zoom. And in my experience (on iphone, iPad and Android phones and tablets, though admittedly not the new retina MBP) the current gifs look pretty bad when blown up.
I think you've fallen for a placebo and somehow prefer the blurry resized images to the "pixel-perfect" originals.
you can include "hi-dpi" versions of images, which a retina-display device will search for, before searching for the original image, e.g upvote.gif will cause safari to check for upvote@2x.gif (or something like that - i can't remember the specifics)
assuming HN follows this convention (and given the number of iOS/OS X users on here, i would be surprised if it didn't), then he will be receiving the high-resolution image, and hence it will look "pixel perfect".
Once again, I find myself surprised by someone's lack of surprise. Do you really think Hacker News uses hi-dpi images? My mind would be blown if it turned out it did. The HTML here is very old-school.
Note also, that the zooming that is done on mobile browsers, and (as I understand it on most modes for the retina MBP) doesn't simply multiply by 2 like native apps on the iPad/Phone so even if you provided a double resolution image then it would be unlikely for it to be displayed "pixel perfect", but the increased resolution (in image and screen) would help to hide any resizing artefacts better. That's why using Unicode characters is a better idea, when possible, as they are vector and scale nicely, particularly so on high-dpi devices.
I think you've fallen for a placebo and somehow prefer the blurry resized images to the "pixel-perfect" originals.