The immunocompromised part comes from the fact that hyperglycemia in diabetics is thought to weaken the immune system. Lou did this to himself when he ate himself into diabetes in the first place, and then continued (continues) to eat the very foods that caused the diabetes. But Lou never goes outside, and he only gets sick from eating his tendies too fast, so I doubt that he's actually facing any issues regarding immunosuppression.
Lou is just salty because he has to walk around the store. His fat-chariot does not await him when he arrives at Walmart, and how dare someone else be using it! News for you, Lou: walking around Walmart is good for your diabetes, moron. Stop with the scootie-puffs.