From your other responses, this is a system issue not a problem with the website.
Lemmy.world’s code has this font list for sans-serif: system-ui,-apple-system,“Segoe UI”,Roboto,“Helvetica Neue”,“Noto Sans”,“Liberation Sans”,Arial,sans-serif,“Apple Color Emoji”,“Segoe UI Emoji”,“Segoe UI Symbol”,“Noto Color Emoji”
I’d use the dev tools to check which font is being rendered. I’m on Windows so I get Segoe UI, which I find entirely acceptable.