I am very confused about the point you are making. I thought the idea of "body positivity" is that each person should be able to decide for themselves how they look, dress, and generally present themselves. Adele has chosen to work out more and adjusted her eating habits and as a result lost weight. How is that wrong?

Every person should be treated with dignity and nobody should be ridiculed. This applies to people that choose to slim down as much as to people that choose to stay at their own weight.