I find it ineffective because most people will see:
A: The magi and a manger scene and not read the billboard fully
B: "Celebrate the reason for the season" is already a Christian catch-phrase, and it'd be easy to mix the two words... rendering it a pro-Christian billboard
Also, believing that Jesus of Nazareth was not born is a very unreasonable conclusion. It is very clear that he existed. Whether or not he was God can be debated.
A: The magi and a manger scene and not read the billboard fully
B: "Celebrate the reason for the season" is already a Christian catch-phrase, and it'd be easy to mix the two words... rendering it a pro-Christian billboard
Also, believing that Jesus of Nazareth was not born is a very unreasonable conclusion. It is very clear that he existed. Whether or not he was God can be debated.