First off, IP cost is no way an indicator to a champions power, I can show you a bunch of examples if you disagree.
Second, Ekko's ult is supposed to reward you if you use it in an offensive scenario, whereas Poppy's ult is meant to "only" manipulate the battlefield to your advantage. Also, Ekko is an assassin who relies on his damage and mobility tools to be successful, while Poppy is mainly a tank who usues her utility to control enemies and defend allies. Which is why a potential ranged execute would make little sense on her.
Just so you know, this doesn't mean Ekko isn't an overloaded PoS. I just want you to undestand the difference.