As another commenter pointed out, Pixar rarely pumps out sloppy sequels -- while the motivation to make Toy Story 2 or 3 may be money initially, they make sure that the sequels have legitimate stories and are generally enjoyable. If they didn't, they would quickly become just-another-studio worth only the length of time they can trick parents into buying merchandise based on their intellectual property holdings.
Pixar doesn't do the straight-to-DVD sequel gig. Even when they continue a franchise, they aren't sloppy about it. Even if we aren't really big fans of the "Cars" style or humor, it's evident that there's at least a cohesive storyline and technical quality. Let's remember that Pixar makes cartoons -- usually, they easily skew to adults, but the more 8-year-old oriented Cars doesn't necessary mean that Pixar is losing it; it just means they're targeting 8-year-olds.