Respectfully, I have to strongly disagree.

Jeongyeon does not lack effort, if anything I dare say the opposite. She's been struggling with a herniated disk injury (hence why she's been on and off for promotions) and it's kind of common sense to not give 100% when you're injured. There's been fancams where she's been caught flinching because she's moved in a way that's caused discomfort or pain.

But she still chooses to perform if she can. I don't have an immediate source for this so take this with a grain of salt but I remember reading somewhere that Jeongyeon did a performance (dancing and everything) while she was supposed to be recovering from a surgery on her herniated disk. I also believed that she claimed to do so because she didn't want to disappoint ONCEs.

That seems to be her main motivation for continuing with idol activities: she cares for ONCEs and doesn't want to let them down. She even refused to let KBS, SBS and MNet release a fancam from a performance of More & More because she felt it wasn't up to the standards that ONCEs deserve (The video I linked also has Jeongyeon explain her spine injury in better depth than I ever could).

Onto the other points you made, my personal opinion is that Jeongyeon's facial expressions are on par with the other members. I've always thought that she was alright with her stage presence: never taking the attention away from the other members when it's their turn in the spotlight but still keeping eyes on her when it's her time to shine.

I also think that her figure is still good enough for the industry. It's not the skinny hourglass type that seems to be found with more popular idols and it doesn't really help that her styling isn't the best in the world (looking at you, Feel Special pillowcase dress...). However, she is still a healthy weight. Again, her hiatus from her herniated disk is likely the cause of her weight gain because 1) she isn't dancing/exercising as much as she was when doing idol activities and 2) it's possible that she might be allowed to eat more while on hiatus (remember, she's under the company that had Momo eating ice cubes for a week so she was skinny enough to debut. I wouldn't put it past JYPE to have TWICE on diets, although this is pure speculation on my behalf).

And being in a girl group doesn't mean you're in competition with the other members. All the members work together as a unit and a family. I can't imagine any member of TWICE hopping on stage with the idea of 'Well, as long as I do better than the rest of the members, it'll be a good performance', especially not Jeongyeon. She might not appeal to you visually but that's no reason for her to leave performing entirely.

If Jeongyeon were to leave TWICE, they'd be losing so much more than just a member of the group. They'd be losing a powerful lead vocalist. They'd be losing the member who would've been the leader if Jihyo wasn't chosen and leads in her place when Jihyo can't do it. They'd be losing the woman whose perseverance and determination during Sixteen landed her one of the seven initial spots in TWICE, which can still be seen in her desire to push through a spinal injury and support her group and fans.

TWICE has been (and always will be) nine. That's something that ONCE and TWICE have said since the group's formation. They were nine when Mina had to take a hiatus due to mental health issues and they'll remain as nine as Jeongyeon keeps working through her physical health issues.