It is a skill, not just raw creativity. Most candidates improve once they stop treating brainstorming as a random idea dump and start using a few clear buckets to organize their thinking.
A good answer usually begins with a simple structure. For example, if you are brainstorming growth ideas, you might separate them into customer, product, pricing, channel, and geography. That gives you a path to follow instead of hoping ideas appear by themselves.
Depth matters more than a long list. Two or three well-explained ideas are usually better than six shallow ones. Interviewers are looking for breadth, logic, and a bit of prioritization, not a rushed laundry list. You may also find it helpful to read the case-interview problem-solving cycle if you want a more structured way to practice this.