I think that's unlikely, though it depends on a lot of factors (UBI is a pretty general concept, and the specific implementation and numbers matter). It may reduce aggregate GDP, but that's not the same as making everyone poorer, since distribution matters when determining that. If, to take an exaggerated example, the top 10% of earners have their income multiplied by 10x and everyone else's goes down by 5%, "everyone has gotten richer" according to some kind of aggregate metric, but in reality 90% of actual people have gotten poorer. Most economists no longer believe that these kinds of situations are automatically self-correcting through "trickle-down economics", in which just getting out of the way of wealth-creation at the top inevitably lifts all boats.