I would just find a heavily used, well-loved open source codebase used by lots of people in production. You need to define "good"/"bad" as actual objective qualities you can see in the world.

I would look for places the code seems to be really liked by its users. Maybe its very reliable, or extensible, or fast, or something else. How do they achieve that? Why do the users say these things? How do they measure / focus / make tradeoffs to focus on those attributes?

Then for painful to use software & common painpoints, why does this happen? Is it a fundamental design decision? Is it just sloppy code? Is in just intentionally slow to be more user-friendly? Or hard to read code because the focus is on speed?

It's all about the tradeoffs and intentional choices...