A lot of the emails I get that come from discussion mailing lists tend to be in plain text format, and they look fine. Some mailing lists also insist on plain text as I recall. I think gmail sends its emails in both plain text, and HTML so if I get an email in my organisation (which 9 times / 10 will normally be sent from gmail) it'll look fine. The only HTML emails I get tend to be newsletters from places like The Guardian, and I normally have to open them in a browser because they just don't look right looking at them straight from neomutt.

I don't know about you but I don't tend to get bothered too much when I get sent a HTML email because neomutt will normally use lynx to format it into plain text that can be read from a terminal. Lynx isn't very good for browsing the web but its better for this sort of purpose. I'd, of course, rather people use plain text more for email because I reckon a lot of the HTML emails I see really don't need to have been written in HTML at all. Although the only issue is that HTML emails generated from software sometimes look horrendous in neomutt which is slightly annoying.