The art on the covers of the Monster Manual was pretty rough, but I grade it on a curve because it was produced by hobbyists who stumbled into the world of professional publishing for the first time, were out of their depth in some ways, but tried to reach for the stars anyway. Sutherland probably wasn't cut out to do a painting of the scale necessary for a full-color wraparound that used pretty much every inch of space on both covers, but the fact that they had the ambition to do a full-color wraparound that used pretty every inch of space on both covers is remarkable in and of itself. (The fact that those covers and the binding were nigh unto indestructible makes it more remarkable.)
And while there's a lot not to like about the Monster Manual's cover, I find the three-level schematic composition extremely charming. A more professional publisher and more accomplished illustrator would have never taken an approach like that. They would have gone with a more dynamic, or dramatic image. But the people at TSR's were amateurs coming out of the wargaming hobby cobbling this stuff together as they went, so they approached the (full-color wraparound nearly indestructible) cover to their first hardcover book in this really blunt and functional way.
By the time they published the Players Handbook, they were well on their way to become more professional (and conventional), but for this one book, they were weirdos charting their own course.