。
She's pretty and on top of that, very understanding.
Literal
She [topic-は] pretty [on-top-of-上に] reasonable.
The connector ~の上に / ~上に ('on top of, in addition to') has a literal spatial meaning ('on top of the table') and an extended additive meaning ('on top of being pretty, also...') — this sentence uses the additive sense. The construction stacks one positive trait on another for cumulative praise. ものわかりがいい ('understanding, perceptive,' literally 'thing-understanding is good') is a body-part-attribution-style idiom describing someone who readily grasps situations and other people's feelings — a quietly important social trait often noted in Japanese personal evaluations, perhaps more so than raw cleverness.