Almost 6,000 users voted on the ATM fees poll, a large portion of whom were not regulars in this community. It doesn’t matter if the user is vocal or not, each user gets one vote which counts as much as every other vote.
And if, as Tristan says, Monzo continue to make these polls more accessible, the minority casting their vote will become even less of an issue. Obviously not every user needs to vote, Monzo just needs a representative sample.