Giger’s original alien design is fairly well recognized as the pinnacle of the art, so sequel decay was inevitable. Once you have something perfect, anything you add to that perfection will alter it and by definition make it imperfect. The further the Alien films diverge from the design above, the worse the designs get, sometimes by fractions, sometimes by great leaps. This is not a comprehensive list of all changes made to the design over the years, but a look at the directions other artists took. Essentially, a brief Fall of the Roman Empire for alien design.
Read more on the original article: Design Analysis: The Alien Films