Is she overpowered? Definitely. Sejuani is probably one of the best, if not the best champion in League given her innate tankiness, synergy with an equally powerful item, massive crowd control, and damage scaling off her health.
She essentially "does" everything. The tank aspect of her is an instant win against assassins, and the % health damage puts her ahead of other tanks, leaving the only champions to contest her fighters and ADCs who build BoTRK.
However, that doesn't mean she is unfair in ranked. The enemy team could always ban her or pick her for themselves. If they let you have Sejuani, it is their fault, and you SHOULD capitalize on that.
Besides, there will always be broken champions in League, and one of them will always be the best of them all. WW dealing 1.5K + damage in a combination of physical, magical, and true damage in one ability did not stop people from playing him. Nor did Rek'sai's original Q that gave her more damage than 3 champions put together or Yasuo's original E that did 50% of an enemy's health bar at level 1 stop people from playing them either.
Trying to win in League isn't wrong, it is what should be done to begin with.