Why Scrappy Doo ruined Scooby Doo? ?

He didn't. He's just part of another version of Scooby-Doo, Where are You?, a show different than Scooby-Doo, Where Are You? called Scooby-Doo and Scrappy-Doo that targeted a different market than Scooby-Doo, Where Are You? (i.e., little kids instead of teenage stoners). Scrappy-Doo was introduced on Scooby-Doo, Where Are You? to test how he appealed to younger viewers of Scooby-Doo, Where Are You?, who were actually watching a show that was somewhat inappropriate for them, what with all the latent drug references and so forth, but seeing as how the show was so liked by little kids, the producers thought it smart to actually make a version of Scooby-Doo specifically for them, because teenagers didn't enjoy seeing all the toy ads aimed at little kids and little kids didn't enjoy seeing all the ads not about toys that were aimed at teenagers.