HDMI is a digital signal. Either the message gets there or it does not. It's not the same as an analog signal like RCA where a higher-quality cable MIGHT make a difference.
This is true in an essentially meaningless sense
You are confusing a digital signal like that of a DVD, or sent in an ATSC or QAM transmission, with one that carries a HDMI TV signal. They are
NOT the same and there is nothing inherently perfect or error-free about a digital-to-digital TV signal as some who are uneducated in this field seem to believe. Actually some things, like scaling, are more accurately done in the analog domain. A HDMI signal CAN degrade. The first sign of an HDMI signal failure is digital dropouts which some refer to as sparkles. The most important work of an HDMI cable is performed by the four shielded twisted pairs which carry the color, clock, and sync signals. HDMI transmission line impedance is dependent on the cable's materials and physical dimensions. For a HDMI cable, these are the shape and size of the paired wires, the thickness, and dielectric properties, of the insulation on the paired wires and the dimensions of the shield over the pair.
These seem to the uneducated eye like simple things to control until you spend some time in a wire and cable factory and find out just how many problems there are with wire manufacturing. Wire is never perfect. Impedance control for a HDMI TV signal is important for a very big reason: timing. As impedance varies, so will the time it takes a signal to travel down the cable. So while your digital signal might still be intact, it may also start out as a square wave and become distorted by the time it reaches your TV. The transitions that mark the edges of bits get smoothed and leveled so much that they are no longer and accurate representation of the original square wave, and they look like relatively gentle slopes. Then portions of the signal lost to impedance mismatch bounce around in the cable and mix with these rounded-off slopes, introducing an unpredictable and irregular component to the signal;
crosstalk from the other pairs in the HDMI bundle also contribute uneven and essentially random noise. As a result, what arrives at your display doesn't look very much like what was sent. Saying that a HDMI signal is digital isnt saying that much as far as signal purity is concerned