I agree that Super Castlevania IV just seems to be visually bland. Many areas are pretty much dominated by two colors. Backgrounds are often faded and bland. And the main character is, as you say, a blob of brown himself.
The beginning exterior is brown with some green and some greenish-grey. The stone castle exterior is a very lighter shade of the brown of the wooden drawbridge. The background is an empty sky (excepting a single small moon) that is a single shade.
Enter the gate and you enter a world of…the same brown with bits of green and greenish-grey. At least (and it really is a “least”) the background has been elevated to a gradient, an empty black band above an empty blue band above a grey band that has actual trees! Faded trees, of course, because all the backgrounds are faded. There are more parallax objects, but they are bland and faded like the trees.
Skip ahead, and you have sections like blue-grey stone with reddish-brown stone. Or the green and brown forest. Or the brown and more brown caves. Or the grey waterway. Or the blue-grey stone and green-brown stone section. Or the red-brown stone and grey wall section, which is not to be confused with the later more colorful brown stone and grey stone wall section.
Completely killing everything is the bland and uninteresting tile design itself. Tiles don’t have much detail, and they get repeated too quickly and too often. Objects like columns and logs are created from one or two tiles repeated ad nauseam. Backgrounds really suffer here, as they are all pretty much awful. Either you get a repeated jumble of a handful of awful tiles or you get 80% of the background created from a single awful tile with a few other awful tiles thrown in to “break up” the “pattern”. The color choices are terrible. There is no animation or any feeling of depth to the designs. Everything about it is just bad. Even things that should have depth end up flattened out by the faded and otherwise poor coloring.
It feels a bit like unskilled artists tried to take the (hardware-forced) wonky coloring and limited tile options of the NES, scale them to the new SNES resolution, and then did a really bad job of trying to make them more “realistic”. (Bad enough that NES Castlevania III screens look better than SNES Super CV IV screens.)