Answer:
Yes it is, as long as you don't bother anyone with it. There are hundreds of active micronations within the United States and I have never seen one be dissolved by the US. The fact of the matter is that no higher government cares about these countries, and that's why they're called micronations, so don't expect any country, much less the US, which upholds the right to free speech and assembly, to do anything about them.
I would like to note here that I am a micronationalist that's been running a country for almost a year at the time of this answer. So don't believe anything a non-micronationalist would tell you concerning their legality.