I've pretty much only read mundane AUs if it's a dear & trusted friend writing it OR if the process of mundanifying it is inherently very funny and that is the point [one year for Yuletide someone did a Lion in Winter coffeeshop AU that made me laugh out loud.] I think there are ways to do it that capture the original characters and do feel like they're engaging with the source in interesting and satisfying ways, but that mode is so rare and I have so little interest in trawling through 500 stories about dull-to-me high schoolers to find the occasional mundane AU gem that I broadly avoid it.