Unfortunately most cameras do not provide "clean" HDMI output. That is, the HDMI output contains camera UI symbols like focus points, battery state, lighting, etc. Configuration options to disable those UI symbols are generally not available, at best one can disable some of them. A model with "clean" HDMI is usually more expensive and you have to explicitly look for it as a feature.

Live view readout via USB is something that works far more often than HDMI.