Ask HN: What are some successful 1 person sites that have a very plain design?

Check out Pieter Levels' projects, most notably NomadList and RemoteOK.

NomadList especially only started out as a shared spreadsheet. There's a good writeup here:

https://levels.io/product-hunt-hacker-news-number-one/

I have found this one on the other HN thread yesterday: https://www.cyclinganalytics.com/ No fluff just stuff. Unfortunately screenshots are low-res, but you can check Try link to at least see reporting with author's real rides. Blog also has some screenshots.

I don't know about "successful" but my solo weekend project is pretty plain only because I'm not much of a designer: https://www.nominal.net

Define successful website?

I have thousands of users, a few websites pay for themselves:

https://easy-a.net/

https://projectpiglet.com/