I'm not sure what you mean by "idolize"? If you're asking me why I say more positive things about Nearl than Saria, I guess it depends from the angle.

Nearl being the paragon conducts herself in a way I find admirable, if detrimental (to herself). She's a lighthouse that guides and helps those flailing in the dark. She realized that it's impossible for her to defeat all that plagues the world. Instead, she'll do her best to act as a beacon of hope so that others don't despair. For them to believe that there IS good in the world and that she will fight for those in need, no matter who they are. To be living proof that knights are real, to inspire them.

It's a largely thankless endeavor and honestly, I wish they explored more on how this type of mentality actually affected her negatively. We only have a few bits and pieces here and there hinting at how Nearl's selflessness is isolating, how lonely she is, how being a lighthouse means little if it isn't being maintained by other people. How she'd give her life for a stranger instead of looking after herself. How she basically ties her self-worth to her knightly creed--who is Nearl without it? She's also chivalrous, charming, and has a cute hidden dorky side to her. And in a world as terrible as Terra where most people are understandably jaded and cynical, someone as idealistic with strong convictions as Nearl is a novelty.

Meanwhile Saria gives off the appearance of someone who has her life together, but in actuality she's failed. So. Much. She's disappointed and let down and hurt practically everyone in her life at some point, that it's almost impressive if it wasn't so sad. Whereas Nearl has her admittedly lofty goals, Saria is more pragmatic, but simultaneously selfish. She enabled Kristen's behavior, possesses an arrogance, inflexible and stubborn to a fault, is kind of a bad friend once you think about it, bad at communication and feelings, and would repeat history again as long as it meant that Rhine Labs survived. This makes her compelling, but at the same time, someone arguably worse than Nearl. By contrast, it's easy to write character studies on Saria, but that's mostly because we see a lot more of her "ugly" side compared to Nearl.

TL;DR I like both a lot, but dunking on Saria is way easier. It's easy to see how someone could fall for Nearl. It's also easy to see why Saria fails at relationships.