You think KOTM’s underperformance had to do with genre it’s a part of being niche?

I think that may have been part of the reason. I also think the fact that it received really mediocre reviews from critics (with some, unfortunately, being overly harsh and mean spirited towards the fanbase), may have deterred people from seeing it. That and the fact that it came out close to movies like Avengers: Endgame and the Aladdin remake, which were some of the highest grossing movies of that year, because of that Godzilla was unfortunately overshadowed. I do think the fact that the general audiences aren't as familiar with Mothra, Rodan, and Ghidorah did play somewhat of a role in the movie not doing as well, with Godzilla vs Kong being more successful due to it having more appeal, due to the fact that both Godzilla and Kong are big names. So overall I think it was a mix of different factors that lead to King of the Monsters underperforming. 

SasquaDash Pretty much said it.

At least people knew who Millie Bobby Brown was or this movie would've made less money...

sonictiger

Given that it was mainly the Godzilla fan base fueling this movies success, I highly doubt Millie Bobby Brown had much affect at all on the performance.

I'm hoping we get another Godzilla movie with Mothra in it before the rights to her expire