Is King Ghidorah a mutation?

Nah, hes most likely a unique alien (Only one)

That wouldn't make much sense

how so?

Because so far even godzilla is a species

Yeah, but still, we don't know. You know what? Here's what we should do. We should tweet this entire forum out to Legendary/Mike Dougherty and see what they say. (And it better not be the same quote that states "that Ghidorah is a creature from space who most likely arrived on Earth many millions of years ago.") We need ACTUAL answers so that we could put this topic to rest. (For Monsterverse King Ghidorah, at least.)

they probably won't respond, but it's worth a shot

Well, if Michael Dougherty "re-wrote Godzilla vs. Kong (2020)" (Btw, I'm surprised there isn't a forum topic discussing that. Why?), then I'm sure he can answer this question. (Insert my forum signature here.)

Wait he rewrote godzilla versus king kong, seriously? Not sure whether to be concerned or happy about that

Yes. I'm surprised you haven't heard of it. Look it up. You're welcome.

Does it matter? From the perspective of a human, Ghidorah having mutated in space prior to coming to Earth would have no bearing on our perception of Ghidorah as an alien. It would only matter if Ghidorah came to Earth and somehow we mutated it. 

It's called world building, typically it helps make a movie better

I imagine that some of his origins may be elaborated on in the film proper.

Most likely

World building and spoiling the mystery are two different things. There's such a thing as over-explaining some things. All we need to know is King Ghidorah is a powerful entity from space that could cause our extinction--That's all we've ever needed to know to establish dramatic tension within the movie. I don't really care if he's a unique creature, an alien mutation or the Easter Bunny one day out of the year, nor is that particularly vital information.

MDK 2 (MinecraftDinoKaiju),
The rewrites Dougherty and Shields did were minimal. There's nothing to suggest they did complete rewrites, they just added some thing for sake of consistency:

The Total Film article said, "Dougherty and writing partner Shields have been able to some input into the next film, 'We did some rewrites on the script just to help carry certain themes across and to flesh out some of the characters.'"

This is par for the course in Hollywood. Many screenplays go through several rewrites from multiple screenwriters. The 2014 Godzilla film had 5 different screenwriters work on it, but only Max Borenstein was given credit. David S. Goyer worked on it as well and Drew Peirce was brought in to polish it after Borenstein finished. Then Frank Darabont came in to flesh out some of the character moments.

Well it sets up that more of them exist so it is important

Maybe if they want to set up future installments? But I don't think it's important to the immediate story. If they bring it up, okay, but I hope they maintain mystery to King Ghidorah's origins. We never really found out where he was from in the Showa series/Mothra trilogy and that's a far more interesting and ominous background than knowing too much about him. What we don't know is far more terrifying and lends credence to the myths behind him.

Well the showa Era and Mothra trilogy were not trying to be as grounded like the legendary trilogy need I remind you of flying godzilla in godzilla versus hedorah

But being grounded doesn't mean you have to sacrifice dramatic tension by over explaining things. That has nothing to do with it.

Did knowing ghidorahs origins in the heisei Era ruin the movie, how about biolante, or destroyah did knowing their origin ruin the movie?

I never said it would ruin the movie. I said it would harm the dramatic tension and take away from the mystery. But since you ask:

The Heisei King Ghidorah's origins have been criticized since 1991, so for many, yes--It put a significant dent in the film for quite a few fans and critics. If anything the Heisei monsters are the #1 most guilty for being over explained--Particularly Battra and SpaceGodzilla which many agree are the weakest films of the era.

Meanwhile, look at the original Alien. All we knew about the xenomorph was what we watched on-screen. Part of what made it terrifying was that it wasn't over explained--The mystery perpetuated its terror. Then Prometheus and Alien: Covenant come out around three decades later and piss off half the fan base with some really convoluted backstory. The fact is it never really needed to be explored and worked just fine as it was.