The whole special is extremely bizarre. Since I intensely dislike Casper (and most of the Famous/Harvey canon), I never watched his H-B revival. The few times I've bumped into it, or this special, I was repulsed all over again by his overly cutesy personality that was further magnified by Julie Dees' voice performance.
The whole Casper/H-B marriage seemed misbegotten from the start. The studio probably bought the rights of Casper from Paramount to adapt to Saturday AM animation because of the success enjoyed by CBS' Popeye revival the season before. NBC must've had a lot riding on this acquisition to schedule
two prime-time Casper specials (the other much more appropriately being Halloween-based) in addition to the Saturday AM series- which was a one-season failure. Forcing him into a comedy/mystery/police format served his limited talents extremely poorly. In contrast, the Popeye revival stayed fairly faithful to the conventions of the franchise while managing to comply with contemporary anti-violence standards.
Casper's supporting cast also was completely eliminated. While Wendy, Spooky and the Ghostly Trio weren't especially memorable, one wished to see them rather than the pathetic Hairy Scary (made to sound like Ed Wynn by the reliable John Stephenson). The Trio, ironically, were the comic highlights of the superior 1996-98 FOX Casper revival helmed by Universal which, like most later revivals of classic characters, parodied their conventions while retaining their beloved qualities.
Then to introduce the classic early H-B characters into Casper's universe was even stranger. By 1979 it was common and accepted to see Yogi, Huck, Quick Draw, the Doggies and other vintage creations interacting with each other. But teaming them up with the Friendly Ghost?!

You never saw the gang interacting with the Harlem Globetrotters, Moby Dick, Gulliver, Josie, Godzilla, the Robonic Stooges, the Addams Family or any other established properties adapted by H-B. Every time one of the gang uttered Casper's name (or vice versa), it was cringeworthy. Even Daws Butler sounded uncomfortable.
The type of humor in the original Casper cartoons was fantasy-based, gentle and frequently sentimental- as opposed to the generally edgy, brash slapstick hijinks associated with the H-B gang. Not a good fit at all. Also awkward was hearing the gang sing the studio's de facto Christmas anthem (recycled the following year in the syndicated YOGI'S FIRST CHRISTMAS movie) as provided by the Paul DeKorte singers.
To make matters even worse, the special carried the dedicated H-B laugh track that had no business being, but was used in the Saturday AM series. Maybe that was a tacit admission that NBC felt kids wouldn't find the series funny unless prodded.
It seemed that Hanna-Barbera hit kind of a creative low point between roughly 1974 and 1982. This special, and the series that spawned it, are prime examples.