Cthulhu was in fact punched out by the explosion of the Roebuck
This isn't to say he was killed, at least not permanently, but he was visibly trying to escape the explosion, so it was clearly doing him some harm. We know that he returned to torpor in R'lyeh in "The Call of Cthulhu" after getting a steamer rammed through his head, though whether he did so because he got a steamer rammed through his head is ambiguous. The stars may simply not have been right. That may be the case here as well; all we know for sure from the end credits is that Cthulhu hasn't risen up to destroy the surface world...yet. But it's equally possible that he was knocked back into torpor or forced to return to R'lyeh to heal after being discorporated, and humanity might be safe from him for a little while longer...if Tian Industries didn't look like it was going to insist on continuing to dig too greedily and too deep. Captain Lucien was into Tian Industries' scheme to contact the Cthulhu
The point is not brought in the film, probably because Norah had no time to dwell on such matters, but consider this: We know that Lucien outright lied that Shepard station was not there anymore, and then when Norah gets there she finds clear proof that they found something that made Tian move the drilling to Roebuck's location, and that it was Lucien himself who proposed it. This would bring Lucien's insistence to stay behind into a new light, not stemming from a sense of duty as a captain, but because he felt guilt about sending so many people to their doom.- It is clearly abandoned, so he wasn't lying. For the purpose of saving them, there was no reason to go there. That was his point.
- His exact words were "Shepard station is gone, there is nothing there". Even if you interpret it as the station still being there but that it wouldn't be useful, that too would be a lie because the Shepard had air and electric power. Also, when Norah sees the map in Lucien's locker, there is a VERY ominous musical cue.
- Or maybe it was Lucien who was a lone cultist, and he'd set Tian Industries up to drill at that specific spot with falsified resource data. He only stayed behind to eliminate survivors and ensure his unsuspecting superiors didn't learn what was really going on until Cthulhu's rise became unstoppable.
The monster at the end was not Cthulhu, but Dagon.
GrimDark (Half Off)
has a few videos about Lovecraftian beings, mostly approached from the Conan side of things, and in these lore videos Dagon is essentially the first Deep One, made by Cthulhu to transform humans into something a bit more suitable for his needs. Given the monster at the end looks less like Cthulhu and more like a Kaiju version of fish men the crew had been dealing with, it would make sense if this was Dagon, not Cthulhu. the design of the creature also bears some resemblance to art of Dagon used in GrimDark's videos.