A single Yonko proved to be vastly more powerful than a Luffy who defeated Katakuri, he essentially one shot him, which I believe was a blunder on Oda's part but that's besides the point, it's what happened. King and Queen are comparable to Katakuri. Big mom clashed equally with Kaido for 3 days, she's comparable to him. King and Queen would get brutally overwhelmed in a 1v2 against either of the Yonko. So yes, defeating Big mom in a 1v2 is more impressive than 1v1ing King or Queen. How much more impressive is up for debate, but it's objectively a superior feat.