See also Configure Neomutt on mobile. 
There is one step more; Deltachat mails may edit previous messages. Also, many statements in a chat make zero sense taken out of context. I think youād need to resend all these mails to a Deltachat client to read them easily as a human. Or write your own script to reconstruct the chat from the mails.
Compared to, say, Pidgin, which lets you export chats as HTML or plain text, this is complicated.
ā¦A Pidgin plugin for Deltachat? 
I also suggest that Deltachat could use a standard mail storage format..