She has been for a long time, heavily gated by mana costs and her E was fairly easy to dodge as well before they gave it the instant reactivation. She was pretty strong in lower tiers of play before this, but she was simply outclassed by other poke mages who had access to more damaging, more ranged, and more reliable abilities with lower mana costs. The only real benefit she had over them was the utility in her shield, which I don't think is a bad shield, it's pretty strong.
Now that they have given her some significant buffs, I think she is a little bit too reliable and she is really easy for your team to play around. This results in a pretty high winrate. She might need a little bit of toning down, but I think she will always be above 52% as she is a forgiving champion and her damage and utility put her in a consistently strong spot. Another thing to keep in mind is that she is really good into this meta of melee "juggernaut" carry toplaners as she stops them cold with her Q and E and can reduce their damage with her W. They are really immobile so she is really good at punishing that. If she was against Maokai, Gragas, and Ekko (meta picks more recently), she would have a lot of trouble into them as they have the mobility to lock her down without getting CC'd by her Q and E. I think her high winrate is a result of the meta and her recent buffs combined and that the buffs are significant, but not cases of overbuffing the champion.