Look, it's not about mountains and molehills or any of that nonsense.
Making fun of obese people is borderline but okay, seeing as they do it to themselves and it is a lifestyle choice. Wait. No. Stop. It is. Nobody ever got fat behind their own back; there's a lot of opportunity to pull out of the project. There's no such thing as big-boned (or rather, people who have big bones have them because they are fat, and the body needs to buttress the extra weight and...just read this short article [http://scienceblog.com/43858/overweight-people-really-are-big-boned/] if you don't believe me). There's no such thing as genetic markers that make people more susceptible to obesity; we all have those same markers. The only markers that increase probability of obesity are environmental ones: your parents letting you eat like a hog, adapting rather than responding to the social cues of peers when they make fun of you, ignoring doctors (or not going to the doctors for fear of reprisal) when they tell you it's unhealthy...ETC., ETC., ETC.
Being adopted is not something you can control, and it always leaves a psychological mark that makes you question your own value. This can be dealt with, and overcome, with support from family, and in some extreme cases, therapy...but that doesn't mean it's proper for a major video game publisher making a game rated for everyone to poke fun at adopted kids. Not only is it poking fun at adopted kids, but it's actually insinuating that they are, in fact, defective (the primary emotional worry for adopted children). The primary thing about it all is that adoption revolves around abandonment of an innocent, for whatever the reason, which takes it a step further than just jibing. Whoever wrote that joke in was either an insensitive bully themselves, or adopted themselves, and wanted to show just how cruel insensitive bullies can be.
Imagine if the joke had instead been about skin color. "Alright fatty. Casper fatty. Fatty fatty red-eye." Or, "Alright, fatty. Coon fatty. Fatty fatty swampbuck..." It's the same thing, even if there are no slurs to describe adopted children. It is isolating something about someone that is out of their control and that has probably caused them pain before the age of reason.
Should they get sued over it? No. Should they get bad press over it? Maybe. Should they go in and do a one-day patch that takes the joke out of the game? I think so.
Should they get an oversight committee to ensure their writing staff acts with a level of decorum that renders situations like this moot? Abso-fucking-lutely.
It's just very tacky, Valve. Indelicate, crude, tactless, and just outright in poor taste.