By default, mu4e tries to display the ’richest’ option, which is the last MIME-part of the alternatives. You can customize this to prefer the text version, if available, with something like the following in your configuration (and see the docstring for mm-discouraged-alternatives for details)
(with-eval-after-load "mm-decode"
(add-to-list 'mm-discouraged-alternatives "text/html")
(add-to-list 'mm-discouraged-alternatives "text/richtext"))