
I believe the MonsterVerse’s shift in tone from G14 to GVK can be boiled down to two factors:
1. The general audience’s reaction to G14 - Since people felt unsatisfied with Godzilla’s screen time and the way the human characters were handled, their reaction may have been the reason why Legendary decided to move away from Gareth Edward’s grounded approach and make the following movie, Kong: Skull Island, more self aware and action oriented; to give people what they wanted from the first movie. After KSI embraced a more lighthearted, tongue-and-cheek tone than its predecessor, the next two entries followed suite, even taking inspiration from some wacky fantastical and science fiction elements from the Toho movies.
2. The decision to do a King Kong vs Godzilla remake - Now, the idea of making a movie about a giant ape fighting a radioactive dinosaur sounds pretty silly on paper, and it would be very hard to take a more grounded approach when making it the way Gareth Edwards made G14. So this could be one of the reasons KSI ended up the way it is; a self aware, lighthearted, glorified, action packed throwback to old monster B-movies where a 104 foot ape is smacking down helicopters while 70s rock music is playing in a Apocalypse Now-esc setting. The decision to do a Godzilla and King Kong rematch meant that the filmmakers would have to scale up Kong, which would make for a rather significant departure from G14’s more grounded tone to a more fantastical tone. This change in tone would end up effecting the rest of the MonsterVerse after KSI.

I think you have really nailed it. While I do miss the more serious tone, lightening was definitely the logical decision.

Yeah, for the most part the general audiences and some critics thought that Godzilla 2014 took itself "too seriously" (even though the original was extremely serious), which probably had to do with the fact that the genre isn't as respected as it should be and the fact that Pacific Rim kind of set a standard for big Hollywood Kaiju movies. The general audience responded better to the more crazy action movie tone that Kong: Skull Island had, which resulted in the rest of the MonsterVerse having that tone.

But even KSI has a darker tone, granted that may be just due to more human violence and death in that move than other MV movies, but it feels like now MV is trying to replicate the MCUs tone

@SasquaDash:
Definitely. Because of how different the tone of the MonsterVerse is right now, I also think that people shouldn’t get their hopes up about seeing Gareth Edwards come back to direct another movie. The tone he established in the first movie wouldn’t work at this point in the MonsterVerse.

2014's tone worked for setting up the series, but I feel that there's only so much they could do with the realistic/grounded tone in a series like this. I feel like they would've needed to bring in the the more out there sci-fi/fantasy tone eventually, especially when you bring in characters like Mothra, Ghidorah, and Mechagodzilla. I do hope that the upcoming Apple TV series brings back a somewhat more serious like 2014 though.

We need to get back to serious on the MonsterVerse. And the sillier tone is wasting frankly spectacular actors and actresses who are somehow brought in (like Millie Bobby Brown, Ken Watanabe, etc.)