That's interesting, PGW's description of Phyllis as a 'pill' is used here to indicate that she has an insipid character. A less unpleasant attribute than those usually ascribed to someone who is a pill. Perhaps some drift of meaning occurred. I have found a definition that claims the usage for the US of A:

(Definition from Cambridge Advanced Learner's Dictionary):

pill (PERSON) noun [C] US
an annoying person:
Jennifer was being such a pill today.

I shall give up.