Is there anything I can do to make poorly formatted mails more readable?
Gnus offers you several functions to “wash” incoming mail, you can find them if you browse through the menu, item Article->Washing. The most interesting ones are probably “Wrap long lines” (‘W w’), “Decode ROT13” (‘W r’) and “Outlook Deuglify” which repairs the dumb quoting used by many users of Microsoft products (‘W Y f’ gives you full deuglify. See ‘W Y C-h’ or have a look at the menus for other deuglifications). Outlook deuglify is only available since Gnus 5.10.