The tanks being overcrowded is pretty normal at pet stores. They don't follow a normal stocking plan like we do in our home tanks, they are only keeping the fish there temporarily, so they are often packed in tightly. One other benefit to that is when the fish are crammed together they seem to have less aggression issues.
The dead fish thing, yeah, it frustrates me when I see tanks with dead fish in them, and I usually point it out to someone. As long as the fish aren't all diseased or look in really bad shape though, a dead fish here and there isn't a huge surprise (lack of quarantine procedures and just sheer volume of fish make it inevitable), the workers should really clean them out though.
It's definitely a sign of lacking overall care, though, since a good LFS would make sure that any sickly/dead fish are removed.