Sure, but let's be real and admit that it is also a drug as well. Many people supplement with kratom for medicinal benefits such as it's potential antidepressant, anxiolytic, relaxation, stimulation, and analgesic properties but at the end of the day it holds the potential to produce a decent opioid buzz to the opiate naive, or at least those with low tolerance... and as such will produce a physical dependence with frequent use, something not really seen with otc herbal supplements.
Really it all falls down to your definition of a supplement. At the end of the day it is a pretty pointless question though. Just see it for what it is, a weak opioid with medicinal uses that needs to be treated like any other drug with the potential for addiction. Many see the benefits to outweigh the negatives when it comes to daily use. However it is vital that you know what you're getting yourself into in order to make the proper decision... and that you're not using the label of "herbal supplement" as a way of justifying careless use.