Assuming this is an actual question and not a shitpost, I'll try my best to answer why people like this book. I'll take a guess and assume your grandma didn't like it for two reasons: 1. She was forced to read it in high school, since it's considered a
must read across the US, despite most teachers not really understanding why in the first place (this is the most common reason for why people may hate this book) 2. She's a woman and old, and thus may have a harder time relating to Holden's outlook on the world around him and his overall attitude, both of which are proper of a young coming of age boy dealing with common issues most men face around their teenage years, all the way through their transition into adulthood. The ideal target audience for this book is a 16-18 year old boy, since they're the most likely to relate to Holden's actions and opinions, and may have an easier time reflecting on themselves and their attitudes through him, with Holden holding up a mirror to themselves.
Nothing much happening in the book is sort of the point, Holden is aimless in life, he has no goals, no idea for what to do in his future, he sees everyone around him as a bunch of "phoneys," fake people who put on masks, he sees himself as cool and an exception to the rule. He clings on to the past and things he's familiar with since childhood, pays unusual attention to small unimportant details around the world others don't care much for (like the ducks and the frozen pond), because he isn't interested in, or is afraid of, growing up and having to leave behind all the things from his childhood years, from when he was happier. He pretends to be cool by smoking, drinking beer, going out with hookers and women much older than him, wearing the deerstalker hat because he sees "everyone else around as prey," and overall tries way too hard to lash out and rebel against a world he doesn't really understand and is afraid of, without much purpose beyond lashing out for the sake of it.
Ironically, he's the biggest phony of them all by putting on a dishonest "I don't care about anything or anyone else in life," when the truth is that he does care, that's what he gets at with the final words of the book "don't ever tell anybody anything. If you do, you start missing everybody," in spite of everything, he still cares about and misses all the people he's met through the book. His antisocial demeanor is a mask meant to help him cope with this.
Tl;dr: The book is relatable to young male teens and men who can still relate to once being a cringy antisocial teenager themselves.
This is my personal interpretation of the book, if anyone here disagrees or wants to add anything, I'm open to replies.
Why Jhon Lennon had to die because of this book?
He didn't, the guy who killed him did it for attention. His excuse that Catcher in the Rye had secret subliminal messages telling him to kill John Lennon (nevermind the book was made before the Beatles even came about) was nothing more than an attempt to create a sensasionalistic news story that would be guaranteed to get lots of media coverage because of how utterly bizarre and ridiculous it was.