Most MUAs these days send multipart/alternative messages, so even though most recipients see the fancy HTML version, the message also has a text/plain version that mutt will default to showing and you won't even notice it had an HTML part.

For those rare HTML-only mails, just install lynx and add this to ~/.mailcap:

     text/html; lynx -dump -localhost -force_html -width 80 %s; copiousoutput

This will make mutt pipe the html content to lynx, which will render it and spit back a plaintext version with all the embedded links at the bottom. You'll be able to easily scroll through the message in mutt as though it were plaintext.