Have you played Darius against her? Sion's not a direct counter but he's pretty difficult to lane against. Shen is currently in a bad spot so she does do well against Shen. Udyr, although you're never going to meet him top since he's solely jungle, will pound her in a slapfest unless he's behind. But at the same time, Poppy does fairly well against Zac.
No champion can beat an entire class of other champion, they can fare well against most or only very few members of that class, but because champions are designed to be unique, you can't just pound an entire class.
And she doesn't have only a bad time against Trundle, Gangplank ignores 40% her excessive amounts of armor with his barrels, 100% of her resistances with his passive, and she can't even beat him with a well placed wall charge because his oranges will remove the crowd control, not to mention his super-short cooldown Q makes him half ranged. Oooh, let's go into Quinn. She can stop Quinn's Vault with Steadfast Presence... That's it. Quinn can blind her, pummel her with volley after volley of bolts, the only time I did ok against a Quinn matchup was when I abandoned the lane and lived solely off of ganks and jungle monsters (And that didn't even matter because Quinn got fed off of my teammates). Another! Ekko. While most tanks don't suffer against the Boy who Shattered Time, the second half of Ekko's Phase Dive easily slips past Steadfast Presence because it's a blink and not a dash. Add in his ability to outmaneuver out of Heroic Charge hotspots, percent health damage, and the ability to undo Keeper's Verdict all by pressing R and you have a matchup Poppy'd rather not see. And let's look at Irelia, she hits Poppy where it really hurts: right through Steadfast Presence’s bonus resistances in the form of true damage. While Irelia can’t Blade Surge through Poppy’s W active, she can stand toe-to-toe with the Keeper of the Hammer and trade basic attacks. Having extra armor and MR below 40% health is normally enough to keep Poppy in the thick of a slapfest, but true damage ignores those fancy numbers, leaving Poppy in a dangerous spot. Oh, and better hope you don't wind up against the dead man walking. Graves gets free resistances from standing next to you and he's ranged! The guy can sit on your face, deal tons of damage, and take none. Yeah, if you see him going top, you better make top lane Draven meta or you're gonna lose. And Kayle, oh my gosh, don't even get me started on top lane Kayle. Pantheon can outpoke her, outduel her, thank goodness for Steadfast Presence or you'd have no chance here. I could go on all day against people that Poppy struggles against and I should know because I main her and I've met them in top lane, sometimes the other way around, too!
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Most of them have control over when and how they use their shield. Poppy is kind of forced on her when she auto attacks when its up, on top of her shield being able to be destroyed (unless she killed a minion with it). So really...Poppy's have more counter play in her shield than most of the other champions who can be played top lane.