Last time I checked, Mordor in Lord of the Rings is the worst place you could ever travel to, so it's funny to see the ironically titled 'The Happiest Place in Middle Earth' t-shirt.
The hilarious 'The Happiest Place in Middle Earth' parodies Disney's large, enchanted castle. So instead of the majestic architecture, you get the Sauron's tower complete with the eye as it looks at you with evil. Below is a phrase that says "One Does Not Simply Walk In." Look at the arc and you'll notice it's the lettering that you see right inside the ring from the movie.
The 'The Happiest Place in Middle Earth' shirt is clever parody that even those who aren't Lord of the Rings may enjoy. Sam and Frodo might not enjoy the irony of the shirt's title though.