By default, the great Mail User Agent MU4E prefers Plain Text mails over HTML. This configuration can be overridden via
mu4e-view-prefer-html, but there's probably few of us who would do that.
However, you might still see a whole lot of HTML emails. And when you check if they have a plain text version, they might have one! There's a reason for that. MU4E has a 'HTML over plain text' heuristic with this official rationale:
Ratio between the length of the html and the plain text part below which mu4e will consider the plain text part to be 'This messages requires html'...