I do not think that Elizabeth was being catty in the least. She was politely straightforward and honest. To have been otherwise would have made her a complete booby.

Darcy first observes that Mr. Collins is fortunate in his choice of wife. Translated, this can be interpreted as "How did such a pompous idiot manage to marry at all, let alone someone who seems to be sensible if not intelligent."

Elizabeth replies that Mr. Collins's friends should be very happy for him that he managed to find (perhaps) the only woman with sense who would have married him and not made his life a misery in doing so. Elizabeth is fully aware that anyone with more than eight functioning brain cells would find Mr. Collins a complete idiotic bore.

Elizabeth may (at this point) question Darcy's morals and his character. She has never disparaged his intelligence. To pretend that Mr. Collins is anything but what he is would make her seem equally idiotic.

Lizzy then observes that she doubts whether or not marrying Mr. Collins was really a good idea on Charlotte's part. Since she has already said/implied as much to Charlotte face-to-face, no harm can come from repeating the observation to someone else.

In my opinion, if Elizabeth had stated that Charlotte made a good or wise decision she would have looked to be as silly as Mr. Collins.