I'm an uncool dad but thought the check mark was like, this account really is the famous person the account claims to be, like Brad Pitt or Oprah. Now it's always someone I've never heard of.
And maybe not too similar but the mayor of my city only has a couple tens of thousands of followers but is definitely known by everyone who lives in it and yes has a blue check.
How famous is famous enough isn't a question with an obvious, or single answer.