A pee bottle for Kilimanjaro is by no means a necessity for your climb, but it can make it easier and more efficient to dispose of your urine in certain situations.
It depends mostly on your personal preferences and how your body works. Also, keep in mind that a pee bottle will take up space and weight in your luggage.
To help you decide whether a pee bottle is a must-have for you, you can ask yourself questions like how often you need to pee, or, do you mind leaving your tent when it’s cold and rainy at night?