Depends on your definition of "mainstream" I guess, but picoCAD[1] got some attention outside of the PICO-8 world. Edit: Including here on HN[2]!

[1]: https://johanpeitz.itch.io/picocad

[2]: https://news.ycombinator.com/item?id=34101251

I believe the game Celeste started as a PICO-8 prototype