Technically speaking, Sion doesn't stand for Noxian ideals. When he was alive, he was a hero, then he died a hero killing Jarvan II. However, when he was resurrected he turned into a bloodthirsty creature that had an insatiable appetite for slaughtering and literally leeched off of death. Standing at around 9-10 feet tall (he's very, very big), Sion can trample hundreds of soldiers at once, not feeling a single thing while they scream for agony, their death fueling his rampage. He's like that secret weapon you can't control, Swain just lets him run rampant killing everything in his path, and it is not human in the slightest. Noxus stands for fighting, displaying strength through battle, fighting until death. Sion may reflect the killing side, but what he does is far from honorable and disgusts even Darius, the Butcher of Noxus.
Poppy is extremely strong for a yordle, though. She's shown to be capable of ripping through materials harder than steel with her hammer strikes, and has displayed that she possesses a much greater strength and spirit of even the most noble of Demacian heroes. However, while she can fight many things, she cannot kill what does not die.
If these two were to fight, here is how I imagine it to go down, lore-wise:
: "What is that thing? He looks like a man, but his eyes are glowing red and there's that weird hole in his stomach..."
: "All troops, we are going to tactically withdraw! Fall back! Release him!"
: Storms out of cage door, charging at the armies of both Noxus and Demacia, tearing all the soldiers to bits while screaming
: "If I don't stop him now, he'll kill everyone!" Pulls back hammer for a blow
: "You... I'll kill you first. Then I'll use your hammer to grind Demacia to DUST!"
: Strikes Sion very hard, causing him to reel back and fly several yards away
: "Pretty sure I heard his neck snap. He should be dead now."
: Moves neck back into place with a loud CRACK "I'm not finished yet."
: "Ergh! Take THIS!" Jumps at Sion with an aerial strike
: Holds out palm, catches Poppy from her head "I enjoy this sound all too much."
CRACK
drops Poppy's lifeless body on the ground, then proceeds to slaughter the rest of the army
. Where are you, Katadaddy?)