What do you want to do? If you want to do a text only template, you should just type it up in Word or Notepad and copy it into the actual email.
Email Templates are in HTML to keep the email structure in a way that is easy to update copy or images, but not break the branding or layout.
As a follow up, Marketo does have a Text Only option when you create an email. Use a Blank Email Template and then check Text Only to activate.