Let me just get this OT section out of the way; it's good that they're aware of it and trying to do better with regards to objectification of female characters. I'm a straight male and a lot of the designs irritate me massively. Oversexualisation =/= quality character design. I'm looking at you, Nidalee/Evelynn/Miss Fortune. Having one female character based around being purring, scantily clad sex kitten? Fine. Three is too many, and there's a lot more than the ones I could be bothered to name.
Now, onto Sejuani in the game.
You're all wrong. She's incredibly powerful in the hands of a good player, particularly at mid-ELO play, since people massively underestimate her. The problem is that she requires a completely tailor-made rune and mastery page, and she simultaneously needs gold early to achieve her build options at the right time and is too weak early to secure kills without good cooperation from team mates. She's also vulnerable to counter-jungling.
That said, in the hands of a good player, a mid-late game Sejuani has, bar none, the best CC in the game. Her build options are also pretty flexible, since she doesn't need any help with CC or mobility, thanks to her Q, so you can turn her into a slow-burning damage machine or a tanky powerhouse, as long as you know what you're doing enough to get off the ground in the early. She's really hard to play early, and there are other people who do her job with much less effort, but if you can use her, she's a ridiculously strong jungler.