Anonymous wrote:OP - They don't like you, so stop wasting your time.
This. I have found with inlaws (and others) that if they don't like you, everything you do is wrong/offensive. But they are more than forgiving/tolerant with the ones they do like.
I believe it happens in the workplace as well. Someone will complain up and down about a supposedly small, annoying thing a coworker they dislike does, but they will forgive the big, egregious things their coworkers who are also friends do.
If I were you, I'd actually keep doing what you are doing. If you do thank-you notes because you feel it is the appropriate response to a gift, then send the thank-you notes -- even to the ones who think it is pretentious. Don't let their pettiness change who you are.
It would be different if the behavior that offends them were actually offensive/inappropriate behavior. But it sounds more like they feel threatened by you/don't like you. If you suddenly stop sending thank-you notes, I promise you that you'll eventually hear that there was gossip about that --- "I got Larla a really nice baby shower/Christmas/birthday present, and she didn't send me a thank-you note. She sends them to everyone else!"
The best defense against people talking sh*t about you is to decide who you are, who you want to be, how you think it is best to behave, and stick pretty closely to that. Let them say what they will say. If you are confident in your actions/behavior, then don't let them change who you are.
As a side note, I like to send thank-you notes. My parents never made me do it, so it isn't even that. I just think that it's a nice way to acknowledge someone gave you something. I also like to get thank-you notes, because it makes me feel like the person took the time to acknowledge that they received a gift, as opposed to seeing gifts as things they are entitled to.
I don't care if people think I'm rude/pretentious/whatever. I think people who don't do something like a thank-you card (or phone call or email or SOMETHiNG) when they receive a gift are likely some combination of entitled and/or lazy.