Pop culture mashups can be tricky. When they're done well, they can bring a smile to your face, trigger a warm glow of nostalgia, and draw your attention to insightful connections between two very different things. When they're done poorly you just wind up with two pop culture references layered on top of each other with no rhyme or reason, like a Dane Cook joke.
These cereal box mash-ups are usually the worst, probably because cereal mascots are personality-free empty vessels by design, intended only to help you distinguish a box of Fruit Pops from a box of Corn Holes at a glance. There's very little humor to be found there and most attempts to find it are doomed to failure.
Which is a long-winded way of saying that I'm not sure why this shirt is supposed to be funny. At least it's a nice blue color that's under-represented in my wardrobe.