I don't think you understand what the word monster actually means.
And I think you are completely baselessly hyping up Zoro and/or underestimating Sanji.
Sanji isn't even at their level, how in gods name do people seriously think Sanji is even remotely ready to take on a powerhouse like Queen who has been one of Kaido's top commanders for decades
For the same reasons people think Zoro can take on a powerhouse like King. We don't know at what level is Sanji (nor Zoro). You saying that he is not at their level while he never had the chance to show us what his level is is totally baseless.
There is nothing Zoro has shown which would suggest he couldn't defeat King.
So Zoro is a monster who can beat a powerhouse like King all of a sudden because he had two fights with weak opponents (Hyozou and Pica) which he won easily and Sanji is not a monster because ever since the timeskip he only trashed Wadatsumi and had two very short skirmishes with a lot stronger opponents than Zoro had (Vergo and Doflamingo).
Zoro has the power and skill to put him down - if he can cut down a mountain size Pica.
It would've been weird if he couldn't do that at this stage since we have seen how powerful Mihawk slashes are. I don't see why you think Sanji couldn't have done the same (with his kicks instead of swords).