The texture paint and noise texture on my creature shows up in material preview mode, everything seems to be correct, but it does not show up in rendered mode or in the final render, or maybe it is there but it's super pale. I'm not sure; my eyes might be deceiving me. Any idea what I'm doing wrong or why it's so faint? Thanks in advance.
I believe the Material Preview mode will show you the "pure" image texture without performing the full shader material computing.
On that note, I notice that you are trying to mix the image texture which seems already quite subtle in variations, with a similar color (the one on the bottom input on the Mix node) using the noise texture. It does not seem surprising that the final render result is very faint.
Try a more vibrant color in the mix node to see the difference. Also, a good practice is to use a Color Ramp on the output of the Noise texture to control the contrast (moving controls left and right.)