There are much more powerful ultimates out there. Are you referring to the self-cast or the enemy cast or the combination of both?
The enemy cast is a point and click 1.5 second stun - but it is only avaliable once every 130/105/80 seconds. The self cast puts her in stasis - which means they do not take any damage or cc and cannot be targeted by abilities and cannot intercept projectiles, etc BUT they cannot do anything for the duration either. - for 2.5 seconds, healing her and deals the same damage as the enemy cast to all enemies in the area.
Both create an area around the target for 3 seconds that slows enemies and deals 150/250/350 (+70% AP) magic damage.
The enemy cast can easily be countered by Quicksilver sash.
The self cast is pretty much zhonyas hourglass + a heal. And anybody can buy zhonyas hourglass. Sure, this is one of the more versatile ultimates out there. But there are far more powerful ones out there. You are a new player you say, well as you experience the game you will find that it is not OP. Also, how many games have you played against her, and how many as her?
If either are less than 5, you can't really judge properly whether it is OP or not.