I do think that the Palmers are an odd couple and the fact that they're married such a short time and behaving this way is interesting.

But about Mrs. Palmer being plump and the idea he can't be attracted to her because of it- I don't think plump meant the same thing then as it does today. I think maybe back then some extra weight was a sign of good health, that the person was well fed (and perhaps in a primal way, healthy enough to carry babies). :-) I'm thinking here too how Harriet Smith is described as plump, and Emma particularly admired her beauty.

Also, when Elinor is analyzing Mr. Palmer in Ch. 20 she thinks ...His temper might perhaps be a little soured by finding, like many others of his sex, that through some unaccountable bias in favour of beauty, he was the husband of a very silly woman -- but she knew that this kind of blunder was too common for any sensible man to be lastingly hurt by it.

So I think Elinor believes she is pretty.

I agree as far as Mr Palmer being a bit of a caricature. What kind of idiot affects being more of a jerk than they actually are? Elinor thinks that it is common though... It was the desire of appearing superior to other people. The motive was too common to be wondered at;

So if the being a jerk is an affectation, maybe deep down he does care for his wife, but publicly she also gets to be a recipient of snottiness.