Cirno is not broken in any meaning of the word. She's nice for inflicting SPD down or PAR on a boss early on, and inflicting PAR on randoms for a bit longer, but these are her absolute only uses, and other characters drastically outclass her in them (Her PAR is very weak compared to many other characters)
Her damaging and survival abilities are both abysmal, which means she has no use outside of status effects that other characters can inflict as well or better then her. She's good early on, but not later.
That tier list looks overall about right, although it's hard to tier due to the fact that character usefulness scales. Minoriko is VERY good for the maingame while not very useful in Plus, and Wriggle is awesome in the main game until... around beating Yukari, while still being totally usable until finishing the main game. And then useless bottom-tier in Plus. Yuyuko and Eirin (more Yuyuko) work the opposite, starting to shine in Plus as they get the MP to spam their big nuke, suddenly becoming rather tanky nukers. Definitely moreso for Yuyuko, who is a great Winner party choice. But due to the amount of bosses who resist SPI... using her earlier can be slightly problematic, sadly.
Also not sure I'd stick Maribel into low-tier. She's not that bad, and can have utility in dealing decent damage and simultaneously having a chance to inflict a good debuff on the opponent, sort of like Reisen, and has some degree of self-sufficiency with her self-buff, as it can be sort of awkward buffing switch-ins sometimes. She can stay out to take a hit decently enough after buffed, so it's viable for her to spam her low-delay skill out for debuffs as well, although Reisen might be better in that aspect. I'd consider Sakuya low-tier before her, at the least.
It's not a big issue though. As long as you keep in mind each character's one or two specific best uses and how they scale during the game, you can figure out whether they're helpful in your party or not, as the mid-tiers can (nearly) all fit quite helpfully into your party as long as you need what they're good at.
In other news, if Maribel, Kanako, and Yuka didn't have such obscene SP costs, they'd be fun to use in the maingame :/ They each do have a single-target affordable skill, but... all their rest are super SP-costly, in many cases on skills that aren't even very strong and would mostly have utility as your go-to normal random trash spell.