I think it also shows a very minor flaw in the open source / pull request process: Expecting casual and amateur contributors to work at the same level as a professional who works with a given codebase on a daily basis is unreasonable.
In my day-to-day job, sometimes I can fix a bug in an hour, but then I need the rest of the day to update tests, avoid regressions, and think through my fix to make sure it's long-term maintainable / readable / performant. Even more important, if I'm working with code that I don't usually touch, I have to make sure I follow the expected conventions that everyone else expects; and I have to be much more careful about assumptions that I haven't learned yet.
So, a 1-hour fix easily balloons into 5-15 hours!
Perhaps the easiest way to encourage new contributors, who are assumed to be casual, is to encourage incomplete and partial fixes that the main team completes?