These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Though considering that the catfish-man looks quite like a slightly higher-budget version of the monster in "It's Alive!" by Larry Buchanan (he of Attack of the The Eye Creatures fame,) it might not be all that original to begin with...
Even Leopold himself points out that his monster form looks nothing like a catfish.
What an Idiot: A defining moment of Walter's stupidity; taking a dune buggy into a swamp where it can be stalled instead of following the other characters on the dry roads to the villain's lair.
To be fair, at this point the characters weren't 100% sure that Leopold and the fish monster were one and the same. Walter tracks the radioactive trail of the monster while the others go right to Leopold's hideout to confirm that the two are the same person by ending up at the same place, or he was hoping to head the monster off at the pass.
Of course, the sheriff tops this by forgetting he has a gun.
"A giant walking catfish is killing people? Well, it probably doesn't have anything to do with that super-creepy scientist who wanted to turn men into fish."