No it's not.
Why would you think lesser of someone simply because they're fat? Because they're gluttons, do little in the way of exercise and are unhealthy? There are a ton of skinny people with similarly awful eating and exercise habits, but due to the fact that their bodies handle food in such a way that a heavier persons does not, they're visually thinner despite being every bit as "overindulgent and lazy."
And how do you know that the person isn't in the middle of a weight loss transformation, such as the 170+ pound weight loss I experienced here:
I received my fair share of negative words while I was in the middle of this 3 weight loss journey, despite the fact that I was likely eating significantly more healthy and working out a ton more than the people who levied them. Hating someone for something as trivial as outward appearance is so fucking stupid IMO.