"Should I not have?" Sirius replies with a hint of genuine concern. "That is what they're all for, isn't it? I mean, if I'm not to use magic, then what am I to use?"

He's largely being facetious, his tone just a little teasing, amused by Kieren's mild horror. And maybe he did make a bit more of a mess than strictly necessary, but he honestly hadn't done it on purpose. It's only that cooking requires quite a few tools, it seems. And Sirius doesn't quite yet know how to use all of them. He's trying, at least.