The cryptocurrency market is still relatively young and can be influenced by events that impact overall market sentiment. If a major stablecoin like Tether were to falter, it could lead to a decrease in overall confidence in the cryptocurrency market, potentially causing broader market declines.
Depends on how shaky it gets. Every now and then its price gets decoupled from the dollar, but that's only a small negative. I'm honestly afraid to even imagine what would happen to the market if usdt goes down halfway.