Translates to: "not super duper." A purposely mediocre-sounding rebuttal to use on someone expressing enthusiasm for a mediocre event. Can be used sarcastically or merely ambivalently.
"Hey, let's go out to Greasy's and get hot dogs. How's that sound?"
"Mmm. Poop-er dooper!" (w/ sarcasm)
"Pooperdooper." (w/ ambivalence)