Weight loss should not influence your plans to lose weight IF the reasons for weight loss are to get back to a healthy BMI.
Proper application of Botox is used to weaken hyperactive muscles causing active wrinkles on certain parts of the face such as the crow's' feet, forehead and frown lines, alleviation of the gummy smile and alleviating smoker's lines and the banding of the neck muscles (platysma).
While a large weight loss may cause a mild deflation of certain, fat parts of the face (such as the cheeks and jowl areas) it will not improve or worsen the areas of wrinkles Botox is usually used in.
I would recommend you get your Botox from a qualified practitioner when YOU want such lines smoothed away. No need to wait.