There are enough humanoid aliens. One reason the GL Corps appeals is that, though there are a lot of humanoids, there are some non-humanoids. Starro as a giant starfish can be silly, but can also be awesome, as Grant Morrison and even Keith Giffin proved (his appearances in Justice League Europe were quite serious and awesome.)
However, I'm not worried. This will later be retconned to say that Starro the Barbarian--with a starfish on him, obviously controlling him---is in turn being controlled by "real" Starro. That the real Starro figures it will take a super-strong humanoid to defeat other super-strong humanoids, and is using Starro the Barbarian as a ploy, a walking target. When Starro the Barbarian is defeated, we'll eventually find he too is being...controlled.