+1 to all skills is pretty nice, and Dr. Fern can't take Crystalline Elder Sign, but this card is moderately expensive (although Carolyn has good economy, made better by targets for horror healing), slow to get moving (a whole turn of actions to get to full), and your ultra-valuable ally slot is filled with soak that you can't rely on too much. This, coupled with a signature weakness that usually ranges from alarming to deadly, makes Foolishness an expensive add to a Carolyn deck. Probably best to stick with the "box" signature set.

I think this is an underrated ally for Carolyn.

Although if you look closely, you might realize, why this cat is unbelievable great. Bear in mind, Carolyn's main ability and this little text from FAQ "Other card abilities or game effects do not resolve with the altered game state in mind; only the indicated ability/action." - you get:

If healing two horrors from Carolyn, but actually removing it from both her and her cat, you get two resources, instead of one. Clarity of Mind helps with that))

As a side note - FFG always makes better versions of the personal cards in the books :)

One other thing. You can give your partner investigator this card with Teamwork and heal horror from it as if it were you giving your partner even more resources and a chance of +1 to all skills. And that's totally thematic - like a cat would take your troubles away.