system
Given the following post, respond as if you're commenting on Reddit:
human
[X-Men] How does Jean Grey ever get defeated?

She's a more powerful telepath than Professor X (if I'm remembering right), she has some seriously good TK, and also wields a cosmic force. What's stopping her from ending all conflict by just reading everyone's mind to find out whatever she needs to know, mind-controlling everyone, and/or immobilizing all her enemies with TK?
gpt
Well, she really hasn't had a lot of time to be super powerful.

In her early years, Xavier installed mental blocks to prevent her from using TP (instead, she was only about a 10-20 tonning TK user) and shortly after he released those she was dead for a pretty long while (though not really, the Phoenix placed her in a cocoon and posed as her up to the end of the Dark Phoenix Saga). After her resurrection she was only mildly powerful before her powers started to grow again and she began calling herself Phoenix for a few storylines, then Onslaught happened. Then shortly after that she died again due to ~~Marvel editorial~~ Xorneto and remained that way *again* for like a decade. Recently she's been resurrected again and gave the Phoenix the bird. 

So yeah. She's either been dead, possessed by the Phoenix, or a decently powerful telepath who fights other telepaths/people who have some way of getting around telepathy for most of her appearances.