Ugh, I hear you. My son sits in a fairly boring Navy/Light Blue model. Snoozer!
I guess it is tricky with boys with colors and any other colors besides blue, grey, brown-ish tones, and any kind of patterns.
When I look for boy prints for Car Seat Poncho, I have to be really careful. Can't be too babyish. The window for "cute" (bright colors, whimsical animals, cartoony images) seems to close at about 18 months. After that, it's gotta be "Lil Man" -- sports, trucks, planes -- but again, not too "fun" or it will slide into babyishness.
Had a few "cute as button" boy prints for boys that took a loooong time to move, but the "manly" ones just fly. Tried plaid, but plaid is tricky because you have to waste a lot of fabric to get the lines to match up.
I think maybe moms are wary of investing in a cute print that their sons (and dads, who usually step in hard when it comes to their son's things not looking girly) will come to dislike. But I agree, I don't see how a touch of a cowmoo or footballs here or there would be such a big deal.
It would be nice to have a print option vs solids, so it would show fewer stains!