Yes if you run Text Only emails as a separate campaign.
No if you send regular emails. The problem is that tracking Opens is done via a 1px graphic inserted by Marketo at run time. Since not everyone downloads the images, only some people are counted as Opens, so that data is largely directional.
Text emails won't provide any feedback to Marketo, so there's no way to know that.
So if you ran the campaign with an AB test and had some people get the regular HTML/Text and the others receive Text Only, you could do some testing on CTR....but NOT on Opens. You could compare Deliverability too, but I'm not sure how different these would be.