Clever is as clever says in this book. Ella's a witty talker, and we can tell because Char's usually laughing at every other thing she says. For instance, after Ella tames the ogres using her mad linguistic skillz, Char asks how she did it. She responds:
"I told them about finishing school and they began to snore." "Truly?" Char stared at me, then laughed. (15.33-34)
On the flip side, characters who are well and truly stupid sound like it when they talk. For example, we hear from Olive:
"Hattie has five and a half trunks, Mother. And I have only—" Olive stopped speaking to count on her fingers. "Less. I have less, and it's not fair." (7.45)
We've been told that Olive isn't the brightest crayon in the box, but her speech (and writing, which is full of terrible misspellings) confirms it in a colorful way that helps move the story along.
We're told Ella is clumsy, but we also get to see her in action. During a conversation in the kitchen, Ella is helping Mandy dry the dishes. Mandy cautions her, "Watch that bowl!" The next thing we see is: "The order came too late. I got the broom" (4.68-69).
This descriptive strategy is more effective than simply saying over and over again that Ella breaks things; instead, we see how broken dishes punctuate her conversations and are just a part of her life.
Stereotypes exist for a reason in this book: ogres will eat you if you give them the chance. Gnomes can occasionally tell the future. Elves are generally chillaxed and like to eat plants. Giants are friendly. Fairies can bestow awful "blessings" and can also perform small magic like Mandy does: mending broken dishes (a helpful skill to have while Ella's around) and manufacturing magical household items.
Humans, in contrast, display a ton of variation in character despite being of the same species. Ella's dad and stepfamily are as selfish and cruel as starving ogres, while Areida's gentle kindness would put her right at home among the elves. Olive is as dumb as a centaur. On the flip side, not all fairies are as air-headed as Lucinda (thank goodness for that). So we'd advise thinking about "type of being" as more like guidelines than actual rules.
Since Ella's the main viewpoint character and doesn't share the obsession with mirrors that other fairy-tale characters have, we don't actually know much about what she looks like. We know she has black hair and starts out short for her age, but that's about it.
So when we hear about the physical appearances of other characters in the book, it usually serves to characterize them according to Ella's perspective. When Ella first meets Hattie, she smiles, "showing large front teeth" (3.18). Olive also looks unpleasant: "The furrows of a frown were permanently etched between her eyes" (3.20).
Surprise, surprise: they look unpleasant and act unpleasant.
Physical appearances can also differentiate characters who are, well, different from the norm. Areida looks different enough from the other girls at the finishing school that Ella gives us a detailed description: "Her dark hair was plaited into many braids that were gathered and woven into a knot high on her head. Her skin was the color of cinnamon with a tint of raspberry in her cheeks" (9.39). This is in contrast to most Kyrrians, who seem to be paler, going on the example of Dame Olga's "pasty white" skin (3.12).
We don't advise going around judging people by their appearances in the real world—but if you're in Kyrria, it's probably not a bad idea.