Far, far, FAR too wordy for a billboard.....as much as I agree with the sentiments expressed.
Too wordy... yeah.... maybe if the words were so small you had to be right on top to really see them, but the L-I-A-R were huge & would look like the only letters until you got close.
-—L—————
-——I————
-———A———
-————R——
While it would need to be a real billboard, the biggest impact would hopefully be on the internet.
Need the outrage factor... LIAR might do it.