Out of all the Star Guardians shown, she's the only one with armor on and her Star Guardian lore says she's the fierce soldier of the team, all this does says she is a combatant.
I checked the splash art again just to be sure, no legit armor. Also, if they wanted her to be the fierce soldier of the team and not just another kawaii, super-powered, anime girl, they probably should've focused more on "fierce" and less on "non-threatening".
Big Ugly Clunky Grey Armor of Blandness
This and this clearly aren't bland and boring.
Yes, She's a Tomboy but she's still a Girl.
Ah, so because she's female, that means she must absolutely adhere to the pathetic stereotype that has glued to all women everywhere? No, she's stubborn-to-a-fault, humble-yet-determined yordle warrior who will find the hero of Demacia or die trying, that's nothing like the aforementioned stereotype that you seem to like.
do you think she really needs to wear a lot of armor(like Garen/Dyrus who are normal human-beings who needs a lot of armor to survive battles) when she has a superhuman body with super-strength to back it up
Yeah, she should either wear a good deal of armor, or have skin made of solid rock in order to look like a tank, since she doesn't have the latter, she gets the former. The only reason she doesn't don heavy armor like Garen and Darius do is because, lore-wise, she travels a lot and thus wears the more practical medium-armor that's durable and light.
(Don't forget the fact Yordles are somewhat fairies, so magic also plays a part in it too, which maybe the reason why she qualified to get a Magical Girl Based skin)?
It never once said in the lore that yordles are fairies. It did say they evolved from fae creatures, but take one look at Pix (not the Star Guardian one, obviously) and it's clear that Runeterra's faeries and classic storybook fairies are very different. And with that logic, Ziggs and Heimerdinger should look all magical and glittery too, because they're evidently "fairies".
Alistar only has two skins with him in armor and he's primary a tank.
And he's also a humongous, hulking minotaur. Because Poppy doesn't have such innate resilience to physical damage, she'd have to wear quite the amount of armor to shrug off hate like he does. Zac has no skins with armor, but because he's a massive goo-man, he doesn't need (or probably have the capability) to wear armor.