Actually was pleasantly surprised by 'Scooby Doo and Guess Who' on the whole. Sure, some of the guest stars are misses and some mysteries are better than others (true for all the previous Scooby Doo incarnations though), but when the guest stars hit they were absolute joys and the best episodes are quite brilliant. Even lesser outings are still watchable. Was not particularly impressed by "The Fastest Fast Food Fiend" on first watch, both in regard to guest star and mystery.
"The Fastest Fast Food Fiend" didn't do it for me on rewatch either when rewatching the whole of 'Scooby Doo and Guess Who' not too long ago. There are good things certainly, and luckily they were good things that pretty much all episodes of 'Scooby Doo and Guess Who' had. It is also though one of the worst examples of a guest star that is poorly written and used and when you're not a fan of said guest star in the first place that adds to the not so good experience. There were other episodes that didn't completely work with the guest star but had a good deal to like with the mystery and character moments. "The Fastest Fast Food Fiend" doesn't have those really.
Beginning with the good, the animation is colourful and atmospheric, with the driving action boasting some visually very clever and energetic visuals. The driving action is pretty thrilling and where the episode most comes to life, just wish there was a little more of it and less of the childish Jim Gaffigan humour. The music fits well with the episode's and show's tone and has a good deal of energetic character.
Voice acting is very good, especially Matthew Lillard as Shaggy. Shaggy and Scooby are still timeless characters, having lost none of what makes them so charming and funny, as is their endearingly goofy yet also relatable chemistry.
Sadly, "The Fastest Fast Food Fiend" could have been much better. The mystery is very thin to the point one doesn't remember it and is also very predictable, the solution was not a surprise at all and felt derivative. It is pretty much swamped by two primary things. One is the writing, or certainly with Gaffigan, which is very childish here and not in a funny or easy to like way. That quality is present throughout and does not feel like Scooby Doo and more an animated Gaffigan stand up act. All the humour centered around the fast food is so overdone and excessively gross out (never thought that would be said about a Scooby Doo episode in a franchise full of food jokes, and mostly clever, endearingly goofy and funny ones) that it put me off fast food for a while after first watch.
Even worse is Gaffigan himself, who dominates too much, is very annoying and nothing about his personality is likeable (too smug and egotistical). He is also a huge jerk throughout to the rest of the gang and for reasons not ever obvious, even in his interaction with Shaggy and Scooby he is unnecessarily mean. Say what you will about the Ricky Gervais and Steve Urkel episodes, which were also brought down by guest star over dominance and Urkel was also annoying, but at least those episodes had amusing moments and above average mysteries. The balance of guest star and gang is completely wrong too, far too much Gaffigan and Fred, Velma and Daphne (all three bland here) are practically side lined. The villain is only merely okay, and mostly pretty uninspired.
Overall, very average episode with one of my least favourite guest stars. 5/10.