Oh, don't assume you know anything about how birthday cakes work in my family, just because I can understand this point of view.
In my family the birthday cake--the entire birthday party, but definitely the cake--is a huge production, with a great deal of planning, sketching, downloading images from the Internet to copy, etc.
I have always asked my kids what they wanted, and I think my own 4yo asked me if she could have a round cake, with white frosting and pink flowers, well in advance.
But when someone asked my son once what they should get him for his birthday, he was offended, and he said, "I'm not supposed to pick out my own present, YOU are!"
(and even then, my kids understand the value of a surprise--they may say that want something on their cake, but sometimes they expect the actual finished cake itself to be a surprise; and my DD has insisted that I can't come peek when she's putting the decorations on MY cake. She's "treating" me with her decorations.)
So I can understand both the stereotypical look that a little girl might really want for her cake, and I can understand that she might have trouble conveying what she wants if she was simply asked, "what kind of cake do you want?" (to me, that's batter & frosting flavors, not decorations) or "what do you want on your cake?" if she doesn't quite know how to put her mental vision into words. And she might expect the stereotype of a birthday cake--what do they look like in all the children's books? Not like her actual cake, that's for sure! They're white or pink, with frosting curlicues around them, etc. (I'll be honest--that black frosting is pretty unappetizing to look at; and other than sticking the dinosaur on, there's not much decoration.)
She may have thought she didn't need to say; that "birthday cakes" are actually a specific design and all those other cakes were not technically "birthday cakes." I don't know what she's used to seeing, or what the conversations were like on the topic. She clearly had some expectation that was not just "not met" but was deeply "not met."
Sure, she likes dinosaurs, but maybe that cake was so different from her expectations that she also feels, instinctively, that "the essence of her identity" was ignored by the cake maker in favor of something cool the cakemaker liked.
Not to say that anybody did anything wrong.
Just to defend the poor disappointed 4yo who cares what her cake looks like but doesn't think she should have to dictate it.