What does it mean: "Of no mean beauty, ..."?
Mean as in small or poor, a mean person being somebody who doesn't give much. Unfortunately mean has a totally different meaning of 'average' which would be confusing!
So "no small beauty" = "definitely a beauty", the double negative is just for literary emphasis.
Mean has the general sense of being poor in quality or of little value, so no mean . . . conveys the opposite. It denotes something very good and of which the writer approves.