Maybe this is just a miscommunication. I don't know if this tax cut will impacts the spending of shaken consumers. But I'm not aware of anything inherent to supply side theory that contradicts the understanding that a shaken economy suffers from a kind of chicken or egg circle, with industry resisting hiring until consumer spending recovers and consumers resisting spending until the job market improves and around we go again. That's not the full story, but it's reasonable to believe that it's a subplot.
I don't know why a supply sider would disagree that more consumer spending is good for the economy. Internet bubble pops, democrat demagoguery and terrorism have a huge impact on behavior. Behavior of consumers as well as investors is not entirely rational, and when the confidence and activity of either is high, it's good (unless it's fanatically high).