which makes me think she's not really, originally silly and stupid.

Her husband is interested in politics, and has to behave in a manner very unlike himself while doing his job.
Perhaps the pent up frustration (making everybody like him :-D is let lose at home and on poor Charlotte, who then is the recipient of that side of his made worse due to suppression.

I see Charlotte actually trying to make up for her husbands unfriendliness and ends up overdoing it.

CH 19:
We see her attempts at trying to get her husband to come out from behind the newspaper but fails.
She tries to cover his behaviour with 'droll' remarks of her own.

"Mr. Palmer rose also, laid down the newspaper, stretched himself, and looked at them all round.

"My love, have you been asleep?" said his wife, laughing."

Now who does that - stretch, I mean!!! Wife is so embarassed she comes out with her droll remarks - and 'laugh'.

When the sisters refuse the invitation to visit Cleveland, Charlotte later asks Elinor if she likes Mr Palmer. I think she's afraid the invitation was refused because of his behaviour.

He embarassess her (like how Mrs Bennet did).

Her comments have been those of trying to either cover up for her husband's ill manners, to cover up her own unhappiness at this, as well as her embarassment, and trying to be polite to her guests under stress and pressure.

"Oh! dear, how beautiful these are! Well, how delightful! Do but look, mama, how sweet! I declare they are quite charming; I could look at them for ever." And then sitting down again, she very soon forgot that there were any such things in the room."

She's clearly very preoccupied.
I see a reverse situation here where the wife's character has been damaged by her husband's.

The only unintelligent thing I see in her is that she's not intelligent enough to avoid situations where he insults, to divert conversation to harmless routes, to not feel it is totally her responsibility to create a situation of warmth and friendliness in the absence of it due to her husband.

No she's clearly not up to this great task.