Bullshit.
He didn't defeat the Tarasque. There aren't "more of them". There is only one Tarasque (per world) and it cannot be permanently defeated.
I'd hazard a guess that what he just defeated is a small parasite that lived on the Tarasque's back.
There is only one reason why someone takes one of the strongest lifeforms from another's continuity and then uses it as a punching bag in their own: It is a flimsy attempt to big up their own characters. It is the sort of thing done by people who write Mary Sue characters.
Are you familiar at all with fantasy stories? They love to use names of mythical creatures all the time, even if they play by different rules. Do you get just as mad whenever a fantasy story has a species of wolves known as Fenrir even though there was only one specific creature named Fenrir in Norse mythology? As long as we the readers understand the power scale of the creature within the setting of this particular story then that's all that matters. In this case, the tarasque is shown to be a creature that a decently strong party of adventurers are fully capable of killing as long as they are properly prepared. Is it power fantasy-ish for one to get taken down by a single undead and his pet rat? Sure, but this is nowhere near the worst I've seen in that regard, and I feel your intense criticism being lobbed at the author for this is largely unwarranted.
I also think it's extremely important to point out that the author very likely
is NOT referring to the D&D Tarrasque, but instead the very creature
D&D itself ripped the name from - the French creature known as the
tarasque (note the single "r" and the lack of capitalization because this isn't a singular creature). Not only does the translation use this "single r" spelling, not only does the visual design of the creature in this manga match much more closely with the French tarasque, but I think the most important distinction to notice is that the French tarasque and the one from the manga are both capable of breathing poison. Compare that to the D&D Tarrasque, which does not have any sort of breath weapons at all - though there is a Pathfinder version of the Tarrasque that gets a Godzilla-style beam attack, but the point is that it has never breathed poison. I think you've mixed up two entirely different creatures, and gotten unreasonably mad when one isn't being treated like the other.