I honestly don't think it's bad at all since it'll trigger if you use Heroic Charge on an enemy, regardless of whether or not you pin them. It's good tankiness that can help you win an all-in.

That being said, if you do bring Aftershock and stun the enemy with your E, which you will normally be trying to do, you won't really benefit from the bonus resistances since they'll be stunned for much of the duration. Against Ranged champs in particular, I don't really see Aftershock helping too much since a lot of their damage will be random poke and autos while your Aftershock isn't up anyway. If you do trigger Aftershock, the ranged champ will likely prioritize kiting away rather than attempting to fight you with your bonus resistances, making them kinda useless.

Poppy can usually trigger Grasp of the Undying easier than you may think since you can trigger it with Iron Ambassador from range, and Heroic Charge places her into melee ranged of her opponent. So while Aftershock isn't a bad choice, I think Grasp is more useful in most situations.

Thanks!