Why is 'her long fingers, as thin as lampposts, moved up the piano keys' and 'winking like a car indicator light, he said "hello." ' not appropriate? can
you me come up with an alternative?

The point of similes (comparisons using like or as) are to make connections, to remind you of something to make it easier to understand.
So playing piano and then mentioning lamp posts is really kind of off-topic and doesn't connect well, neither does winking and car indicator lights.

D. prior knowledge