Fred from Scooby-Doo seemed to have a nice physique, but not once in 10,000 episodes did he ever take his shirt off.
Times have changed. In Scooby-Doo: Mystery Incorporated (2010-2013), he flexes at poolside.
Anime Boys with Their Shirts Off blog displays the shirtless boys and men appearing in a huge number of animated tv series, everything from adventure to comedy, and even some toddler tv. Did you ever want to see Dora the Explorer's brother Diego with his shirt off? Or Bill from Curious George?
There's a lot of Japanese anime, like The Legend of Korra and The Daily Lives of High School Boys), but also a lot of Western cartoons, everything from Phineas and Ferb to Johnny Test.
There are even a few oldies, like these golden-haired preppy types (from Beverly Hills Teens and Attack of the Killer Tomatoes, respectively).
See also: Saturday Morning Muscle