Would gaining some weight following a jowl lift affect the outcome in a negative way?

I can imagine how a weight gain is 7 pounds would have significant influence on a facelift. Major weight fluctuations can indeed generate changes. I don't think this would be the case in your situation.

When a patient gains weight, depending on age and activity level, it can be a combination of an increased volume of muscle and fat as well as water retention that occurs.  Even if the weight gain is almost entirely fat, the increased volume of the fat cells is usually distributed over many different regions of the body.  You do not state your age or weight in your email. Obviously a weight gain of 7lbs in someone who weighs 100lbs is more significant than a weight gain of 7lbs in someone who weighs 300lbs.  Nonetheless, it is a safe bet that the entire 7lbs of weight gain has not settled exclusively in your jowls.  We tell patients that the results of surgery usually remain stable over long periods of time if the patient's weight also remains stable over that time. We know that weight gain can affect people's facial proportions, volume and contour, even in the absence of surgery. In your case, I don't think the central issue for you is stretching of the facial skin affecting your lift.  Rather, seven pounds over three weeks is a pretty good clip of weight gain.  Were you to continue at this rate, you would, over the course of a year, gain a very significant amount of weight.  So the most cogent argument for you getting your weight under control is to avoid becoming morbidly obese and the attendant risks of Type II diabetes, heart disease, stroke, and many other diseases that accompany this condition.  So, yes, losing that weight will probably not only improve the results from your surgery, but, more importantly, your long term health and quality of life.

Relax. 7 lbs. is not a problem but be prepared for some days results looking better than other days. That is part of normal healing and at 3 weeks post-op that is probably what you are observing.