Just because one game has the same dumb idea doesn't justify you using it as well.
Is it a trope that you disliked in Wasteland? Or Wasteland 2 or 3? Or Planet of the Apes 2?
Nope, worshipping something that can just wipe all of humanity is stupid no matter the setting, time period, whatever. This is not the same thing as the Legion trying to enforce their power by using methods used by an empire from ancient times because at least there's evidence of that working. Worshipping something that has shown to be nothing but pure destruction that led to nothing but misery is fucking stupid no matter what.
And your point is honestly even more ridiculous when you realize that i'm looking at it through the setting of Fallout and not OURS. Ours hasn't been obliterated by atomic bombs yet unlike in Fallout. Fallout should be the last setting to have a faction worshipping fucking atom bombs.
Ever heard the term "fear of God"? It's a pretty common trope throughout all of human history to be fucking terrified of the things you worship, because whatever you worship is more powerful than yourself. That can translate into terror, and into love and adoration. The God of the Israelites, and now the majority of humanity, literally killed everyone except like 8 people, constantly kills his own people, and promises the literal coming apocalypse.
Caesar's Legion is a rational ideological project organized by a single rational subject. That is not what a religion is, religion is arrived at irrationally, ecstatically, and by process of revelation. Cults are not rational. I would also note that, for 99% of it's members, there is no real rationality behind the Legion. Only Caesar really understands the full shape of things. Granted, though, there is more rational articulation by the average Legionary of the purpose of their project than there is by an Atomite or any other cultist.
You're looking at it from the point of view of someone today, who is afraid of their world being destroyed, and knows that worshipping the atom bomb is nonsensical. Or mabe you're looking at it from the point of view of someone after that, but one who is intimately aware of the details of the modern world, and supremely rational about it. I don't doubt that plenty of people would rather worship the golden age of man than the thing that destroyed it, but the alternative is not ridiculous when you consider that
there is no other world.
To a lot of people in the Wasteland, all there is is the world as they know it, nothing else. Anything else is j ust a story. What does seem to be well agreed upon is that the world as they know it was created by the atom bomb, that the atom bomb is supremely powerful, and what's more that their is a singular, physical embodiment of it. It's just too powerful of a psychic symbol that somebody would not worship it. It's just another way of coping with the state of the world.