Does exercise (especially weightlifting) negate the effects of Botox on the Masseter muscle?

You should be able to continue with Botox treatments to your masseter and weight lift. Botox will cause this muscle to atrophy. In between treatments, the muscle will start to strengthen if you are clenching. Best of Luck. 

Botox jaw aims to cause atrophy (reduction in bulk) of the masseter, the jaw muscle, to slim the face. Eating chewy foods, teeth grinding and jaw clenching is in effect exercising the muscle which may cause hypertrophy (increase in bulk). Whether or not to continue with weightlifting is largely a personal choice

Although we usually recommend a break
from strenuous activities (i.e. exercise) for 24 hours after Botox injections are performed, after that you should have no problems weightlifting. It should not affect the results.

You should not be straining your facial muscles while you are working out, or you are simply overexerting yourself. Your facial muscles do not help you lift weights, so you don't need to pull your face like this. Many bodybuilders have square jaws because of steroids, which create angular jaw muscles. It's not the exercise that's the problem, it's how you are pulling your facial muscles while you work out. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."