to be frothy in simple terms is to layer on the flirt a little too much.
everyone likes a little bit of froth every now and again but after a while it takes away from the overall drink if it is too much froth.
also may be used in a positive light for something being cool.
to be frothy in a good way is just to be a cool person.
question: did you go on a second date with that guy??
answer: No, he was way too frothy.
reply: oh, ew.