Do not make the decision based on one bad mock or the feeling that you should be more confident by now. Interview preparation is noisy, and even strong candidates can have rough practice sessions close to the real interview.
Instead, look at the pattern across several realistic mocks. Are you consistently missing core skills such as structure, math, and synthesis, or are you mostly dealing with normal variance and nerves? That distinction matters.
If the fundamentals are clearly not ready and you have a legitimate opportunity to reschedule, asking may be reasonable. If you are broadly interview-ready but just uncomfortable, more time may not solve the underlying anxiety.
Either way, make the call quickly. Spending three days debating whether to reschedule is usually worse than choosing a path and using the remaining time well.