Cover fashions, even more than other features, are largely decided by the buyers from the big retailers, like BRU/TRU, Target, Walmart, Amazon, etc. The manufacturers produce a very large number of concepts, most of which are rejected by the buyers. Sometimes the buyers don't like any ideas or samples provided, in which case it is back to the drawing board. The best sellers are almost always neutrals like black, grey and tan, presumably because people buy them to match their vehicle interiors. So, the buyers tend to insist on those boring types of fashions and the cycle continues.
Retail buyers also indirectly determine other things. For example, they may give better shelf space to products that have wider age/weight ranges or those with higher margins (ie made in China) and can influence other design choices too.
This isn't only for child safety seats, but also for strollers and various other products. So, especially with fashions, it's probably not the manufacturer that is mostly to blame if you find the choices to be boring or lack boy/girl or celebrity fashions.