R’ Avraham Gurvitz shlit”a told me that this ideology comes from R’ Nachman miBresliv, and that the Nesivos in his tzavooh speaks very sharply against this idea. He said, (apparently like R’ Yaakov Kaminetsky), that a correct version would be Simcha Gedolah Liheyos BMitzvah Tamid.
It seems that the Machloikes is how to explain ???? ?? ?’ ?????, “Serve God with Happiness” – whether the happiness is a means of its own to serve God, or whether the happiness is the ideal way of serving God (i.e. Serve God happily)