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All he did was squeeze through the hole that the rats were too big to fit through. And it was just luck that he survived and happened to grab onto another rat. It didn't take any special skill.
Well, the book describes the situation better in my opinion; while he is viewed as important, he isn't quite given the same reverence that the movies bestow on him. He made it possible for them to escape, and given the situation they were in, it was unlikely they would've made it without him.
As far as fandom love for the character, I think the fact that he's such a blank-slate character-wise means it's easy for people to project the qualities they want onto him.
I find it interesting, and it makes sense, that in the fandom he's often portrayed as having some sort of anxiety problem. He couldn't tell his wife about NIMH because of his fear that she'd be hurt by it. This is really his most interesting canonical feature because other than that, yes, he's a blank slate.
I would not go so far as to call Jonathan a blank slate. After all, he not only helped the rats to escape from NIMH, but both book and the movie mention his role in drugging Dragon the cat (especially important when the rats needed to be out at night).
You don't really know much about what he was like, personality-wise, though. You can infer a few things, sure, but there's a lot of room for interpretation and even outright guessing.