I use aerc, which is similar to this, but written in Go. Most of these TUI mail clients have a configurable filter option for different MIME types. I've found that this program does a great job as an HTML to text filter:

https://github.com/jaytaylor/html2text

I think the classic approach is to use something like lynx or w3m with networking disabled, but I wasn't impressed with the output.

With mutt, you can tell it to pass HTML through an external program (like a text mode browser) to render to plain text. Not immediately obvious if meli supports that, but it is at least tractable.

Elinks for rendering html mail and antiword for rendering word documents inside the mutt message view.

meli does pass html messages to w3m by default, but you can change it to anything else with the `html_filter` setting.

In Mutt I auto-view those with w3m. The main limitation is that you can’t reply while preserving the HTML (at least I know of no TUI HTML mail editor).