I think they're awesome, but isn't the game of a rather serious character, meaning it contains only biologically/physically realistic lifeforms? Part of the icons are conceivable animals and robots, but others look like cartoon characters to me.
No, this game does not strive for realism
Anyway, nobody agrees on what realistic would be in the xenobiological realm. Though at least i've clearly avoided the 90% anthropoid biped line-up that you usually see.
I'm primarily motivated by visual necessities: making each icon simple and unique enough that it can be distinguished from all the others at small sizes. The visual techniques required for this have a lot in common with the techniques of cartoon-ifying something.
My secondary goals are alternatively to a) make original aliens, and b) hit most of the popular alien archetypes that many players would want for their custom species.
EDIT: that said, I freely admit the quality and style varies somewhat between icons as I've been trying to find the right ballance between time and quality, detail and readability.