If I were to judge the finished painting - I would say it's good. But the show is not about painting but about performance.
And leaving the audience clueless while paintng upside-down for "x" minutes is not good enough. Watching paint dry is not what people pay to see. And the revealed painting IMO has to be not just good but amazing.
I've seen this routine performed by others. They make a grand entrance with buckets of paint, big brushes, and fling paint with "wild" brushstrokes. They engage the audience with quips and banter. For the reveal they spin the painting, use smoke bombs, or strobe lights -- something dramatic.