What’s the worst-case consequence of signing credit card receipts with a drawing (like a rocketship) instead of a signature?

No repercussion to you, the negative affect (if any) is to the Merchant. If you dispute the legal signature on the receipt, and cite your card was used without your consent, the Merchant would have to bear the proof that the signature on the receipt matched with the signature at the back of the card (or in the event it was missing or illegible) then a secondary picture ID was used to match the signature.

If the merchant cannot provide conclusive proof of this, you have a strong case for reversal. The card company in their right only want you to make sure that the signature at the back of the card is also a rocketship!