Halloween is all about the kids and the fun they have. "Theme in Yellow" is likewise for the kids and celebrates that time of year when they get to be as silly as they want to be. Even the jack-o-lanterns are only "fooling," so there's no need to get super-serious and meaningful in this poem. The pumpkin-speaker just wants us to have our fun and enjoy the scenery—like a kid would.
Kids just want to have fun with some pumpkins in Sandburg's poem and aren't interested in taking things too seriously. We know—shocking, right?
Nice try, junior. Kids may want to have fun, but it's the adults who can truly appreciate the beauty of those "orange and gold clusters."