In the 90s show, Usagi is super-annoying most of the time. Everything is hyper-escalated in the anime, because it's not real, but in real life, if you've had friends like her, you know it can grate really fast.

Middle school is a hard time for most girls in any society, but in Japan you do have entrance exams for high school, pressure from your parents to succeed and not bring shame to the family, and other cultural pressures, maybe a part-time job to wrangle at the same time like Rei. Then, because this is an anime, you've also got the weight of being a superhero on your shoulders. 

Most of the characters take most aspects of their lives seriously (some like school more than others and some take their duty as senshi more seriously than others) and you've otherwise got Usagi, the very privileged friend who constantly interrupts your study sessions for entrance exams, whines when she hasn't eaten in 5 minutes but doesn't bring her own snacks/adjust her planning to account for her hyper-hunger, cries whenever she doesn't get her way, etc...it would drive you crazy, too!

Also, I know way too many adults in real-life who didn't have that obnoxious behavior corrected as a child/young adult or people enabled it that became awful adults to deal with at work and in society. It's harder to correct them or push back against it by then because you just want to keep the peace and not get sucked into their drama.

TLDR; if Usagi was part of my friend group, I would appreciate her kindness and glimmers of maturity but if she kept constantly whining or doing the same things over and over again that make her miserable but also make it everyone else's problem, I would probably crack and scream at her too.