I think part of the intention is that she's not a real cat; she's more similar to the humanlike animals in a lot of cartoons and fairytales etc. Which is fitting since Bandle City itself is supposed to be akin to a fairytale, a whimsical woodland village inhabited by various cute and funny humanlike animals. And canonically, Yuumi is a spirit that just happens to take the form of a cat and have a personality similar to one. Now that we're getting more info about Bandle City and its origins and where Yordles and Yordle-like spirits themselves come from, it seems that the entire realm is a whimsical approximation of the world outside, not completely true to it. As is Yuumi.
Now, whenever Nidalee gets a rework/mini-rework, that's when I'd be a bit more concerned about feline appearance accuracy. Because though she's described as turning into a cougar, right now in-game her cougar form takes the "mountain lion" nickname a little too literally. Or, even if she's a totally different kind of big cat that just so happens to be called a cougar too, at least make the model look somewhat respectable. (I just want a Nid rework DX) But Yuumi looks incredibly cute for me and will suffice for now.
