Underdog 6 (1975)
It took me a little while but I finally tracked down this fantastic Swamp Thing parody within Underdog 6.
One of a few stories within this comic is, “The Mop-Thing!”. The story tells the tale of the Swamp Thing-like muck monster with “hideous strength and horrifying stench”.
Underdog ventured down to the bayou for a vacation but instead gets a history lesson in Mop-Thing. Similar to Alec Holland’s fate, a “brilliant research scientist” meets his fate in the swamp, covered in his own ingenious “soda pop or fantastic new hair tonic”.
The swamp monster ends up making his way into town and after a little confusion, he lands a deal with a movie agent!