In mailchimp there is a feature that allows you to view the number of email recipients that receive plain text (as opposed to HTML) versions of an email. Does such a feature exist within Marketo?
No, I do not think so and it would not make much sense : Marketo does not send either one of the other versions, it always send the 2 in a multi-part message and the email client chooses the one to display.
I wonder how Mailchimp would know this though, unless it really send one or the other version alternatively.
Dory Viscogliosi had a nice idea in another post: if you set your links in the 2 version to have different url parameters, you may know at least how many leads clicked a text version vs a HTML version.