Usagi's delusional confidence about her own importance is the entire joke, and the show knows exactly how funny it is to watch her convince absolutely nobody. The special is basically a highlight reel dressed up as Usagi's fever dream of self-importance. It's pure fan service wrapped in the kind of comedy that only works if you already love these characters enough to laugh at them.
Usagi and Chibiusa stumble into a conversation where two girls are ranking the Sailor Guardians by intelligence, grace, and combat prowess. Naturally, Usagi insists she's the smartest, most elegant, and strongest of them all. Chibiusa just rolls her eyes. The special cuts through each Guardian's origin story, pairing their transformations with their signature image songs. Mercury's brilliance, Mars's intensity, Jupiter's power, Venus's charm, and even Tuxedo Mask's mysterious appeal get their moments. But when the girls finish their list without mentioning Sailor Moon, Usagi can't help herself. She interrupts their conversation to demand recognition. What follows is a backhanded compliment of brutal honesty. The girls gush about Sailor Moon's heroism but don't shy away from her actual flaws: she's clumsy, she cries constantly, and frankly, she seems unprepared for the job. Usagi's forced smile as she accepts this mixed review is comedy gold, especially with Chibiusa watching the whole embarrassing moment unfold.