She just looks at things differently.

Like in the episode "Snow Job" where she's selling cosmetics, I actually understood her when she was originally talking to Janet and was explaining how one was her lucky number. It made sense to me even though Janet was struggling in her head to see what 1 and 10 and 15 had in common. I don't remember the exact line and how it went. She maybe didn't have common sense, but she had several moments of uncommon wisdom. She thought up of things the ordinary person wouldn't.

It depends which dialog :)

Chrissy: I know where we'll get money. We'll do a garage sale!
Jack: We don't have a garage
Chrissy: We'll buy one!
Jack: Where are we gonna get money to buy a garage?
Chrissy: From the garage sale!

But, I do remember a scene on the couch where Janet was furious about someone (Jack?) and Chrissy said something that sounded dumb. When Janet went "What??!" Chrissy looked at her and said it differently in very intelligent way which made Janet realize that she was the one wrong. Too bad I can't remember the episode.

She maybe didn't have common sense, but she had several moments of uncommon wisdom. She thought up of things the ordinary person wouldn't.

Suzanne Somers called this Chrissy's "circuitous route to logic." It wasn't considered common sense or even quick sense, but it did make sense (most of the time). If anything, Chrissy probably complicated things more than they needed to be, as proven by her often long and confusing explanations that eventually arrived at a point of logic.

I, too, have had moments while watching the show that I think, Wow, Chrissy may be the smartest one in the room. But sometimes that thought doesn't last long. :D