Well Dick Grayson is the greatest human being alive, so it's not really fair to compare.
Want people talking about Karen? Might help (among other things) to pair her up with a new love interest. Some guy who is super hot, objectified just as much as she is.....he's gotta be able to keep up with her, so prior experience with metahumans would be nice. The guy has to be pretty great, and do something to help his community like a fire fighter, cop, or even another hero. And it might be nice if he was also a legacy of some kind, since Karen's got ties to Superman. Maybe just to play up some contrast this love interest could be a important, beloved member of his mentor's family (since Karen's the black sheep of the Supers).
I'm being real subtle here right? It's Nightwing. He should be the love interest. I'm just using this as an excuse to pitch my ship.
But really, what makes Karen more worthy of a solo than Mera or Fire & Ice or Batwoman (or any of the C-list guys like Booster?) Nothing. But nothing really makes her less worthy either. Who's got a story to tell that'll fill the right niche in the publishing line? Who's got something worth saying? Really, that's what it often comes down to, I think. They'll publish Batman and the big names no matter what. If Aquaman sells like crap, he just gets a new creative team and a new #1. But the C-list? That's often less a case of which character is most viable (because really, none of them are worth much on their own) and more about what creator has the best story to tell. I mean, the only reason Adam Strange is getting a book is because a writer has a story they want to tell with him. That's not a character DC decided to actively invest in; they don't care about Adam Strange they care that their writer has an interesting story. If Tom King had pitched Vibe instead, that's what we'd be getting.
If someone has a Power Girl pitch and DC thinks it's better than the other pitches they've got, it'll get published. I dont think DC keeps a scorecard for the C-list. You're right though, that Karen needs a team book (preferably the JSA) to build momentum in. It seems like she's gotta be visible so people don't forget she exists, but once they remember her they also remember how fun she is and her popularity seems to increase quite naturally.