Quite simply, whoever designs Chiefs apparel just has the worst taste ever. The font, the styling, the slogans. Awful.
One has to think of what makes the Chiefs special or great. I think it's the history, the "original guard" factor. You don't do that overtly, but conjuring up a fifty year old super bowl. You do that stylistically.
The apparel needs to all feel throwback. Use faded colors (also because that bright red is just toxic on the eyes). The clothes should feel vintage - they can range from a war poster in WWII to a beer ad in the 80s. But play on the history, get rid of that terrible block lettering, and literally tone down the colors.
The "Let's do this!" on the back of the T-shirt above is closer, but seriously "Let's do this?" that's the best we can get?
This is closer:
