Again Zoro attacked Doflamingo, the shikibukai who had just unloaded a magazine into Law in front of him. He was against an enemy he already knew was powerful that just seriously injured his friend. I don’t see how you can say he was holding back against Doffy but Luffy was going all plus ultra against fodders when we know for a fact Luffy had a whole new Gear since before the New World
Zoro's words to Luffy not being serious wasn't him saying Luffy shouldn't hold back his power level, it's in response to the fact both of them lost due to sleeping gas/lack of oxygen. He was scolding himself for allowing himself to be defeated like that, as indicated by Sanji's response to Zoro's words.
Zoro is a hypocrite, literally after that scene he proceeds to hold back against Monet despite the poisonous gas and even the risk she poses to his own crew, that's just Zoro's flaw.
Luffy isn't holding back his punches or his speed on randoms.