It varies a lot.

Mobile-only which strictly emphases the content and removes all other distractions can actually be a bonus. I've found some such sites (with appropriate margins restored) to be preferable to many desktop sites.

Mobile sites with lots of gee-wizardry are almost always horrible.

My ideal is actually very close to how TBL's original WWW proposal documents are styled. I have a default "unstyled" stylesheet I apply to most such sites, which consists largely of:

Setting body background to #fffff6, foreground to #330, font size, width to auto, max-width to 50em, and margins to 2em (top/bottom) / 4em (sides).

Setting anchor and heading colors and sizes, and size of standard text elements.

It's a pretty good foundation for fixing a bunch of sites.