Out of All Dumb Decisions in GoT This One Was The Worst (And It's Not Dany's Arc)
Actually, you can even say that Dany's arc was planned quite cleverly.
Though actual writing was flawed, most of the backlash against it comes from people who expected a straight happy end and never considered that a character who successfully solved all of her earlier problems with a whole lot of dragonflame and slaughter might apply the same approach to people we're not supposed to loathe, particularly at the moment soon after much of her life falls apart around her (two out of three of her dragon children dead, her conviction of being the rightful Targaryen heir shattered, the man whom she loves being that rightful heir, etc).
No, as far as dumb decisions go, it is hard to beat Jon Snow's death and resurrection.
What purpose it had in the book and the show, other than to serve as a not-cliffhanger (because no one ever believed that either Martin or the showrunners would really kill him off), a nearly 12-years-long (and counting) not-cliffhanger in the case of the books?
In the show it comes in Season 5's finale. And sure enough, Jon Snow is brought back to life early in Season 6, just like everyone expected he would be – after all, the setting has an established magic power for reviving freshly dead people, and a character, capable of using it, Melisandre, was hanging nearby at the moment when Jon got stabbed by his fellow Night Watch members, who believed that he was betraying the Watch's ideals.
Furthermore, not only Jon's death did not work as a shock moment, because the audience saw straight through it, Jon's death also served no narrative purpose.
Imagine him getting only wounded by the traitor Watchers. What would have changed? Nothing, except that death in the series would not have been cheapened a bit. He came back with neither a mystical revelation, nor any lasting mental trauma (as happened to other revived people, though it is only elaborated in the books).
At the very least, what happened to him was not drastic enough to actually change his behavior. His death and resurrection did not really impact what was going on outside of Jon's head too. You could have expected more reaction from people witnessing these events, either awe of this miracle, or terror, given that rising corpses are something that the Night Watch had seen plenty of by that point, but no.
The whole event was dumb simply because it did not matter.