By losing 20lb after a tummy tuck, the most probable is that you will look better, if you lose weight the fat in the inside of the abdominal cavity, that means you will have a smaller waist. Also it is important to consider the quality of your skin, if the skin has a good tone, that will not be a problem, if your skin tone is very lose, you may have just a little amount of extra skin that may require a revision to remove that excess of skin, but this is not common.
Thank you for your question.It is impossible to predict exactly how your tummy tuck results will change with weight loss. Regardless, I would recommend that you achieve what you consider a healthy long-term weight most conducive to optimal quality/quantity of life. Deal with any subsequent breast changes if/when necessary. Best wishes.