“Hey, what’s da Creature from the Black Lagoon doin’ wid Maude’s daughter?”
Courtesy PTOR, friend of this site and maintainer of the famed Sanctum Sanctorum weblog: a page from Crazy Super Special #88 (July 1982) by Larry Hama and Bob Camp, featuring…well, you’ll see:

I don’t recall Mad or any of the Mad-a-likes ever doing a full-length parody of the Swamp Thing film at the time, since I’m sure the film wasn’t prominent enough to warrant the attempt. So, this may be as close as we ever got, unless someone out there can tell me something different.
Bob Camp’s Swamp Thing caricature is pretty great, but man, just imagine an actual Mad parody of the Swamp Thing movie drawn by Mort Drucker. Almost brings a tear to one’s eye.
Or a parody drawn by Don Martin! “A bomb! Got to defuse it before….” “BAAAROOOSHOOOMOOOMOOOM” “AGGG! AAAAAH! GARGH!” “KERSPLOOOOSH”
Don Martin’s sound effects when Swamp THing grew a new body would have been awesome. Grunch Spaloosh Poit Glit Glorp Schtoik!
I’m udderly shocked that you managed to find anudder appearance of Swampy for me to hunt down. I must find it, udderwise my collection would be incomplete.
Hey, Dave Sim called his parody Sump Thing as well. It appeared in the very first phonebook, issue 25 to be exact. That issue came out in February ’81. Marvel is playing it fast and loose here by making the hyphen the only difference in the two names. Just like we have seen with the X-Men spoof called XMEN (pronounced Zhmen) last week… :)
So Hama and Camp not only invented Mystery Science Theater 3000, but the “dull surprise” gag as well.