Not quite true, if I understand you correctly. Clones are a form of asexual reproduction, not that different in principle from what is observed in bacteria. So plant clones do undergo mutation and natural selection.
http://www.backyardbananas.com.au/faq.html
http://www.daff.qld.gov.au/plants/fruit-and-vegetables/fruit...
And for very good reason: every banana of a variety is susceptible to (eg.) a virus outbreak - there won't be any pockets of genetic variability like you would get with potatoes or citrus. If anything that affects bananas becomes widespread - no more bananas.
Bacteria reproduce in all sorts of bizarre ways, including swapping DNA from other bacteria, editing their DNA, etc., so it's not really related.
I guess if you had simply said that there's virtually no selection pressure, you wouldn't have triggered my nitpick reaction. :)