An impressive accomplishment. Not staggeringly impressive, but fairly difficult and worthy of some respect. Relatively informal.

We organized a meditation retreat and love-in weekend at our house, and that was no mean feat when you consider that Linda's a right-wing gun nut, Mike is a fruitarian anti-vaccination activist, and Molly just stole Elizabeth's girlfriend.

"Mean," in this context, appears to mean "average" rather than "cruel."