does any story become less serious because the existence of tortles?
I know almost nothing about D&D tortles - they weren't a part of my B/X play, or any AD&D play, or my brief experiences with 3E, or my 4e play.
Torchbearer 2e does have
Turtloids in a "denizens" chapter (Lore Master's Manual, p 260):
Clever and cruel, the turtloids inhabit swamps, marshes and wild bogs on the fringes of civilization. A proud stock, despite their hard shells, turtloids feel keenly the barbs of disapproval from other peoples. Their pride leads them to vengeful cruelty.
Turtloids tend to travel in groups of two or three. Should the opportunity arise, they will drag a child into the swamp and drown it out of spite. Or capture travelers who stray too close to the water’s edge. Turtloids adore candlelit dinners and are known to force their captives to entertain them with recreations of the luxuries and riches of the great metropolises in their humble marsh dwellings. . . .
Nature 4 (Turtling, Creeping, Snatching (with the Claw))
Might 4 [this is more than an adventuring PC, and the same as a Bugbear]
Precedence 0
Type: Folk
Instinct: Always creep from behind and watch creepily. . . .
Marshfield: In their home marshes and bogs, turtloids are granted +1D to creeping and snatching tests.
Turtlocracy: Larger turtloid nests are led by a horned turtloid who is Nature 5, Might 4 and Precedence 1. A swamp that hosts many turtloid nests will be ruled by a Great Terrapin who is N6, M5, P2.
Turtloids also figure in some examples of play in the Scholar's Guide, including these ones (pp 38, ):
Anthony’s elf ranger, Thor’s magician and Dro’s thief are digging a ditch to try to flood a turtloid lair. Anthony tests Laborer while Thor and Dro help by describing their efforts. Anthony fails the test, so the game master indicates that the ranger is exhausted from the effort, while the magician and the thief are merely angry that their vast talents are being wasted in this wretched swamp. . . .
While camping in a storeroom in the turtloid lair, Myrgan the thief and Ban the Magician have two checks between them. They decide to pry open a locked chest they found, but they now must take great care as they’re surrounded by lurking enemies. Dro fails the health check to pry open the stubborn container. Luke, the game master, describes the wrenching noise that echoes through the dungeon. A pair of turtloids soon arrive to investigate, ending camp before Ban can spend his remaining check.
I feel like this stuff is trying to push Turtloids hard as serious antagonists. I haven't yet tried to see if it works in play . . .