I think the current thinking in psychology is that you can affect your overall happiness through changing your general patterns of thinking in the most basic of ways. I believe that having a happier narrative on the world makes you happy. And even small things -
this article (naturally with a stock photo of someone smiling) summarizes the notion that even just smiling can make you happier. I believe this is the case and generally force myself to be happy once in a while.
But all this isn't the original question! The question is, is the poster offensive?
I say the sentiment isn't offensive, but the poster is. Not because it isn't true, but because it was placed by management. They think a workplace is improved if all the people are regularly reminded to be motivated and happy by motivational posters.
Well, I know that I'm happiest in an organization when there is strong leadership in which I am valued.
I like workplace posters that remind us to do our fucking jobs. "Perform final inspection on all outgoing shipments" is a poster I like to see in the warehouse.