Not at all! If anything, it’s perfectly understandable, not only because ponyta is perpetually on fire but also because whether or not that fire would burn a human depends entirely on that human’s bond with them. So it’s actually quite natural to be hesitant about that.
However, rest assured that if your ponyta wants you to ride her, she’s very likely harmless. Always be sure to touch her first to ensure that she’s warm but not burning, then mount her carefully. If you’re worried about whether or not she’ll burn you if she’s startled, so long as you keep her calm and guide her gently but firmly, she should be able to keep her flames under control.