Yeah, I don’t like that either. They’re made to be clear at 700px wide — the size of the image on the comic’s main site. But Tumblr shrinks them to 500px for feeds, in a way that loses detail. If you click on the pic, though, it should be visible at normal resolution.
[Shameless panhandling: People who donate $10 or more on my Patreon get to see everything in its full 2800x3400 print-ready glory.]