Happiness is such a personal and subjective thing, I think the message is innocent but the tagline is guilty of maybe promising too much, and is morally questionable. I appreciate Disneyland probably could be measured as the happiest in terms of the greatest amount of people in one place being happy.
The tagline is inherently misleading in a way, as you could not visit Disneyland and other places would be the happiest place on earth, such as home. Even those who have been, happiness is transient, variable, ever-changing and challenging.
Maybe I’m analysing too much?! It gets a strong 3.4/5 as Disney is magic and this tagline compounds their qualitative albeit immeasurable happiness!