It appears that more than one F-bomb gets dropped over the course of Eighth Grade, which means an R-rating is a guarantee.
While a parent might not want a child subjected to that in some cases, the context here is something different.
Kids swear, mostly because they can, and showing anything less than that would be a less than honest portrayal of what the movie is trying to convey.
While movie ratings certainly have their purpose, as it’s valuable for any audience to have an idea what they’re going to get before they walk into a theater, this does perhaps show that sometimes a one size fits all rating system doesn’t necessarily work.