It simplifies a whole civilisation down to a few design styles.
Not entirely true, it defines certain root patterns perhaps but that should not be unexpected given we are talking about a machine race here effectively - the Guardian AI and a few ruins are all that remain of the guardian civilisation as far as we know.
The currently available data we and lore on the Guardians see to indicate they were technologically advanced but that does not necessarily mean they were diverse in their approaches to designs.
It's a classic sci-fi mistake. Humans have thousands of styles. Aliens only ever have one.
Aesthetics is a poor justification for design variation if that is the ONLY reason. In the human case, we have a number of diverse cultures that evolved independently on the most part with different approaches to art, religion, philosophy, and language. At a basic level, there is very little variation in human designs and the aesthetic aspects are normally a characteristic of who is involved in designing/building it and what the core motivations are behind the activity.
Where insect hive (Thargoids) and machine hive (Guardian AI) races are concerned there is going to be a natural consensus and commonality in overall approaches due to the fundamental nature of the entities under consideration. Where other alien cultures are concerned their cultures may not have evolved in as diverse a fashion.
In Sci-Fi, generally speaking (there are some exceptions) any given alien race is often considered to have a single culture - sometimes with different factions but generally speaking if there is a major schism/difference between the races at a philosophical, religious, or linguistic level then they are considered a separate race even if their general root origins are identical. A prime example of this are the Vulcans and Romulans in Star Trek - the Romulans and Vulcans are essentially the same race but they diverged on philosophical/ideological grounds and then evolved differently from each other in different parts of the universe.
The primary justification for humans having more diverse designs is that we have a broad and diverse set of cultures that has evolved on a single planet and mingled over the years. In common sci-fi, humans rarely truly unify into a single culture and in the cases that do go down that route they are normally portrayed as being doomed to not persist as a unified culture.
Human technology is normally positioned as being (at least initially) weaker than alien technology in one area or another but often humans adapt alien tech to work with nominal human tech and in doing so can result in even more diversity (in an overall sense) than we already have but generally speaking how that alien tech is integrated tends to follow a particular pattern. The same rules of thumb apply to any new wholly human tech too, in some cases there is diversity from the start because of different human factions attempting the same work independently of each other.
In ED specifically, we have different manufacturers taking different approaches to ship designs and each of those manufacturers tends to be generally consistent in their approaches across a given range of tech that is designed for a specific purpose. In the case of the human-guardian hybrid tech, there appears to be a single unified group incorporating the alien tech with ours thus (at least for now) the designs resulting from that work are naturally going to be consistent in at least general approach - the focus is on integrating the tech rather than trying to introduce variety for the sake of it.
In short - lack of diversity in aesthetics in a given culture is expected, humans are often considered to be a mess of intermingled cultures (hence diversity in aesthetics) while individual alien factions are generally considered a single culture and different alien cultures are typically considered different aliens even if there is a common genetic baseline.
