I ignore the genetics argument because to me it matters relatively little.
There are many reasons people are fat and "laziness" is just one of them. Society enforcing an unhealthy lifestyle (long workdays mean little time to cook or work out) is as culpable as genetics. The fact that poverty correlates so severely with obesity is as striking to me as the genetic correlates.
There are problems with fat shaming that have nothing to do with why people are fat, though. You don't get to be lawmaker in addition to judge, jury and executioner. Being fat isn't illegal - you've just determined that it's a negative societal trait in some way. Deciding society's rules for yourself (perhaps because you've felt some pressure in that respect yourself) and then enforcing them by shaming those you feel don't comply is pretty sick.
Also, it's hard to determine, in passing, how lazy someone is. You're all gamers - you spend hours everyday moving almost not at all (dat mouse arm workout). Seeing someone in a period of inactivity does not tell you about their lifestyle.