I think microcopy with personality can definitely improve an experience, but a few caveats, which you touched on.
Don’t be creepy. Like any conversation, assess how familiar you can be at any given point in the relationship. In other words, it’s great to be friendly, but don’t overdo it, creeper.
Careful with those idioms. Cute, clever microcopy shouldn’t leave those less familiar with the cultural vernacular behind. Think of those who don’t speak the primary language of your product—or the tragically unhip (like yours truly).
Easy on the snark. It might sound good in the studio, but in real life it can translate as uncaring or rude. I once encountered a third-party travel site where the null search results read, “Looks like you’re not going anywhere!” Yeah, real funny…hahah.
Microcopy is the oft-overlooked sibling in the UX family, and the reason I suggest those interested in the practice really brush up on their writing skills.