Does autoresponder emails should be plain?

Plain text wins mostly because the filesize of the message is smaller and thus the spamfilters work faster and can analyze the data better,which in turn may be a plus if you're not sending spam.
But the same rule applies with the html messages as long as they are small like 50kb in size.

Below is some research data I think answers your question -

1) Adding images (GIFs) to emails reduced open rates by 37%
2) Using an HTML template in emails reduced open rates by 25%
3) Increasing the amount of HTML in your emails reduced open rates by 23%

But the open rate isn't the whole story. The goals for every email marketing campaign are click throughs and visitors to your website, so here’s more data on the click behavior related to HTML vs. plain-text emails:

For the plain-text vs. image test, the image version had a 2.3% lower click-through rate. This, combined with the lower open rate, meant the plain-text version got 42% more clicks.

For the plain-text vs. HTML template with images test, the HTML version had a 21% lower click-through rate. Combined with the lower open rate, the HTML email had 51% fewer clicks.

For the simple HTML template vs. HTML-heavy template, the simpler email had a 5.3% higher click-through rate, which, combined with the higher open rate, resulted in getting 30% more clicks.

This is significant from a performance perspective, and serves as solid justification for the proliferation of non-HTML emails we're all seeing.

Hope this helps.

Test.

I use basic html so that I can use hyperlinks in place of URLs and have tracking.

I monetize 3rd party data and opens tracking is a must!