While it's not my favorite season, I personally enjoyed Supers and I don't think it's bad as everyone makes it out to be. I don't think Usagi is that much more immature than she is in the other seasons like how Usagi seemed to care more about playing video games than the fate of the world in the first season. In one scene in R, Usagi even thought Umino and Naru were taking up pro-wrestling. I can understand being confused about foreign words, but how do you get pro-wrestling out of promise ring? And Usagi is really only immature during the comedy scenes but I think her maturity develops in the scenes that matter. I think there's some great storylines in Supers like Fisheye's sacrifice to save Usagi and the later episodes where we learn more about Nehelenia's past and Chibi-usa's developing relationship with Helios.
There were a lot of filler ins but I think it only seems like more than usual because they had all the bad guys attack at once. Like instead of Tigerseye, Hawkseye, and Fisheye attacking separately like the Shitennou did, they went out all at once so it seems like it's more repetitive even though it's about the same as the other seasons when you think about it. But even the filler ins had some great plots like the episodes where Ami and Rei got their new power ups, the episode where JunJun tries to help Chibi-usa's friend learn how to jump the vault, and Minako's two-timing episode. The ending to Supers is a bit anti-climatic but I consider the Nehelina arc in Stars to be the real ending to the arc and I liked that it made Nehelina more sympathetic and fleshed out. And I loved that Usagi was willing to risk her life for Chibi-usa by jumping off the floating tent so high to save her without a second thought which shows how much Usagi actually has matured. While having the Outers would have been nice, I feel like their seriousness would have clashed with this season too much but the Supers special with them in it was still good and I thought the Chibi-usa vampire special was hilarious. Let's not forget that Supers also gave us Ami's First Love which is one of the best Sailor Moon specials and showcases Ami at her coolest.