I mean, there is a lot of well-animated and well-drawn anime "targeted toward" adults (Cowboy Bebop, Black Lagoon, Samurai Champloo, Space Dandy, One-Punch Man, etc. These are all labeled as "Seinen").
Most American cartoons targeted towards adults are comedies (South Park, Family Guy, Aqua Teen Hunger Force, Rick & Morty, etc), so there's less of a need to make them
look
good. As long as they're considered funny, they'll be successful.