Tbf, I think it's quite an uplifting statement- Either to atheists or religious people (those open minded enough to realise that they're "probably" wasting their time believing in some supreme being).
I say this because the slogan is true! No one can realistically claim that there is a god, but there PROBABLY is no god. So to me, the message is one aimed at all people to just relax and not take things so seriously.
As for the man not driving that bus, I personally think it's pathetic. It's a message aimed at all people and what it says is probably true whether he likes it or not. Maybe HE should "stop worrying and enjoy his life" perhaps??