Are you sure it's not the quality of the video itself? Do you have adaptive brightness on? Do you have keep on while viewing engaged? The video might be dimming because it perceives you are not looking at the screen and is starting to lock the screen. Does it brighten on it's own after it dims?
If it only happens with YouTube videos, I'd guess the problem lies with YouTube. If it happens with other video apps, then it would likely be your phone.
