In botanical taxonomy, Mirabella is also associated with a cactus and a member of the four o'clock family. So perhaps one might look at these names as meaning Belladonna had a rather poisonous personality, and Mirabella was either prickly or lazy, inclined to bestir herself only rather late in the day. As Tolkien says that Hobbits were fond of flower names for girls, I wouldn't be at all surprised if he occasionally had such things in mind.
