Matrimony might still be only a vague thought, but I can see Eleanor approving of Catherine as a companion to Henry. She might be "simple" as in "uncomplicated" or "unworldly", but she's not silly. Could Eleanor perceive that Catherine can deal with Henry's silly phases, and even redirect it a little bit? She isn't the type to marry him, and then ignore him as much as possible (or laugh behind his back). Nor would she make an effort to "reform" him.

To use a metephor, she could "rein him in" from time to time, without "breaking his spirit". I could see them maturing into something not unlike the Crofts of MP.