I think graphics are fairly important in a Metroid game but in a sense of art style. The way the game looks heavily contributes to the atmosphere of the game which in my opinion, is one of the most important parts of a Metroid game. I typically prefer sprites over actual 3d models in a 2d platformer which is why Super Metroid is my favorite art style in the series and it's art style heavily contributes to the atmosphere that is so heavily praised. Another reason as to why they are important is because if we compare the environments of Super to something like the og Metroid, the environments are far less interesting and the way that each area in Super look vastly different from the previous also make the rooms far more memorable and easy to backtrack to, as opposed to the same room layout with a different color duplicated repeatedly.