Maher is right that being fat isn't healthy, and celebrating being fat is horrible.
Maher is very wrong to not point out the societal issues - particularly those in the US since Bill was a kid and Americans were much thinner - that have led to Americans being much fatter.
If you work two jobs just to pay the bills, live in a food desert and what you can buy is filled with corn syrup and milk, it will be VERY hard to exercise and eat right to get/stay thin. Comparing again to Bills childhood: thin people in the 70s are now those who are overweight to obese. 70s chubby Americans are now those who are now very overweight/obese/morbidly obese.
Kids that grow up with weight issues tend ot keep those issues through life. It is the exception not the rule for the chubby kid/young adult to trim down. Usually it goes the other direction. So an entire generation of American kids from the 90s and 00s when neoliberal policies really kicked in are now struggling both with finances AND their waistelines.
Yes, people have agency, yes, people make bad decisions. Societal issues matter more than individual agency when looking at a whole society and not just individuals.
I see Maher's argument similarly to people who blame black Americans in general for being poor. When the deck is stacked so high against you, it is tough to get rich or even middle class. The same American societal problems stack the deck against Americans trying to get and stay trim and healthy.