Does Rejecting an offer of a peace treaty influence warmongering penalties?
An AI nation starts a surprise war against me, and later asks for a peace treaty.
Does rejecting such an offer influence warmongering penalties or upset other AI players?
No, rejecting peace offers does not influence warmonger penalties.
Warmongering penalties are primarily applied when the war is initially declared. That's why the AI tends to mass-denounce the aggressor the next turn. A player can receive additional warmonger penalties for razing cities, and in some cases for capturing cities.