I agree because this is likely true, but also disagree on the grounds that it's not a good argument.
First, i have to make the distinction that Mgqp isnt a 'casual and lighthearted' game, it's a lazy game, the difference being is that one series intentionally keeps things brief and ssurface level with the intention of creating an easy and engaging experience, the latter simply can't actually be bothered to elaborate on characters or world building, mgqp is the latter.
Second, mgqp being a lighthearted game that doesnt take itself too serioussly is wrong, hold on, it's probably true, but its wrong because it makes no sense.
A game, where the main plot is about saving the multiverse from a slow and grueling death because of entropy, where along the way you will be raped, killed, eaten, raped and eaten at the same time THEN killed, can hardly be called a 'non serious' game
Lastly, mgqp takes itself VERY seriously, almost too much, the level of angst some portions of the game have along with the general premise of part 3 along with some art pieces, while very well made, seem like something you'd come up with in highschool. I agree that the fact that mgqp doesn't take itself seriously is prob the intention, but it's actual nature is so contradictory that i can't fully agree with the statement.
All in all, the game's flaws can't be refuted like that, you can easily have a casual but good story, this game is hardly that, and personally, if you came from the original mgq trilogy and expected deep and profound plots, i agree, you're definitely an idiot in that case, none of the stuff that's been discussed in this thread is actually that deep either, which reflects the nature of the game.