恐竜ガール is a natural combination of words, but its meaning is ambiguous.
- A (human) girl who loves dinosaurs. A big dinosaur fan. Like this.
- A wild human girl who lives with dinosaurs. Like this.
- A female child of dinosaurs. Like this.
Despite the ambiguity, 恐竜ガール sounds natural and "interesting" to me as a book title, etc., all the more for its ambiguity.
But I would like to note that 恐竜ガール is likely to be taken in the first sense (ie, "a dinosaur fan") these days. Perhaps many people will take this phrase in the first sense right away. In the last few years, this
○○ガール meaning "(female) ○○ fan" is regarded as "trendy" by some, and I hear many similar expressions, for example, 山ガール, 森ガール, 数学ガール.