A dingledorf is a person who is considered to be goofy, silly, or dingy. A run-of-the-mill dingbat, in other words. We also used a variant form back then, "dingledorfer". The definition would be the same, the "er" just made it slightly more fun to say (at least when you were in Junior High in the mid 80's that is).
Marty is such a dingledorf that it is a major embarrassment to have to share a locker with him!