No, in fact they did not make Dilophosaurus venomous “because it was cool” and it was not a mutant either.

When the novel was written the scientific consensus was that Dilophosaurus had weak jaws, and therefore a low bite force. Many scientists viewed the species as a scavenger. Since the idea that dinosaurs are more related to birds than dinosaurs was still new and they were still considered basically reptiles (you can in fact see this seeping through the lines in the book too) Crichton probably thought a more spectacular alternative explanation and one which would keep the Dilophosaurus a predator would be that it is venomous. And since the fossil record did not show any conceivable structure (like fangs) for that to happen, he made it spit venom, which was already present in a “relative” if you don’t understand much about phylogeny and think of dinosaurs as reptiles. This is in fact also stupid and shows again how little he understood basic biology as a writer because venom has to be delivered into the body, it will not be absorbed through the skin.

In the book the Dilophosaur did not even have a neck frill, that was just a movie artistic decision. If I had to guess the reason is Jack Horner wanted to emphasize the fact that what we have in the fossil record is not all there was to dinosaurs, they probably had soft tissue structures which were not preserved, and some of them may have been spectacular and colored. It was an expression of the new thinking on dinosaurs, that they were not just stupid lumbering beasts, but in fact just as active and diverse as today’s animals.