Disclaimer: This is going to be HEAVILY based around the Rick and Morty fanbase only, but that doesn't mean I hate them all and all other fanbases are better. I don't, they make hilarious r/iamverysmart posts, I'm only talking about the bad Rick and Morty fans because that is what my reply is about. All fandoms at least have that group of people who are jerks to everyone else, and think that they're better because they understand the show and like it, and I know a few Rick and Morty fans that aren't this way, but this is aiming towards explaining why I dislike the Rick and Morty fanbase specifically.
There is a video in a spoiler at the end, I will warn right away there are cuss words, not like every other sentence but they still exist! There is also a clip of showing a female's chest through a whole time, thats why I'm saying its a good background noise to listen to if you get uncomfortable with that stuff. Video overall is decent, its about Fandoms, and lightly skims over a few fandoms that are a bit ridiculous.

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Although, every fan-base can be obnoxious, a lot of them are, but I haven't heard as much about, say, Undertale, as I have the things Rick and Morty fans have done for the sake of their precious show.

I actually only heard of the Undertale toxicity recently, but not being directly involved with that fandom or even really the game I'm not surprised. But I'm not involved with the Rick and Morty fan base, or even the show, I've only heard tid bits and seen a few memes, I guess that could count as a form of involvement but the point is I'm not up to date with the fan base at all and a lot of the things Rick and Morty fans (Among a few others like FNAF, but thats mostly because I enjoy watching Game Theory, and how defensive some of the people can get about FNAF and their theories as to whats going on is hilarious to me) and the things that Rick and Morty fans have done is a bit ridiculous, and wide-spread to the point someone as secluded to fandoms as me has heard of it.

Now don't get me wrong, I'll say it again, R&M (I'm shortening it now lol) is probably a good show. I haven't watched it, I haven't gotten the chance, but my brother and my dad are R&M fans and they like it, so I have no doubt I probably would too. But to give an example as to what I'm talking about with the ridiculousness of the fandom, https://i.redd.it/nsooaqi10scz.jpg. I got a good laugh out of it, and yeah every fandom has one of those guys, but I found that particular fan pretty hilarious. But it demonstrates an issue I find with shows that try to challenge your mind, or look like they're challenging your mind and saying "smart" things, you'll have fans that think that because they got a reference, they have a high IQ, they're smarter than everyone else, etc. The other show I've heard of having fans like this (Though there is many), is I believe the Big Bang Theory. Not sure, but think it was that one.
There is a video that I watched once that went over the ridiculousness of Rick and Morty fans in particular to try to answer why fandoms feel so entitled, I will warn the video has a few cuss words and a little bit of nsfwness (no images on screen really just the guy talking about certain things and there is a clip of a video while hes talking about entitlement that shows a big breasted womans chest for awhile, which was awkward but if you run it in the background its not so bad) so its not for some but it goes over one of the falling points of the R&M name, specifically the fandom. The Szechuan sauce that Rick rants about. Well at some point McDonalds returned said sauce for a 1 day thing and the stocks were low, since I mean its a 1 day event, and had a slightly Rick And Morty themed cover image on the part you peel off, but Rick and Morty fans were all over it, and god forbid you came across a Rick and Morty fan that was also all over this stuff working for McDonalds. Someone got chastised by a Rick and Morty fan working for the drive through because they weren't "A true Rick and Morty fan! Rick and Morty fans don't get the sauce JUST TO TRY!!" It was a nightmare, and worse of all it did put the Rick and Morty fans on the chopping block on social media for being "a horrible fandom for a brilliant show." There are probably a lot of good fans on the fandom, those fans are probably like the 30% of the fans that exist for that show but, it was ridiculous.

TL;DR McDonalds is lucky their stores didn't get burned down over a sauce and Rick and Morty fans, like a lot of fanbases, are quite ridiculous.

I mentioned a video earlier going over Fandom Entitlement to their favourite show, so I'll post it in a spoiler. It brushes over Rick and Morty and a few others but its a nice listen to.

just remember that i'm more of a doctor who fan than u are b/c i painted my face TARDIS BLU!!! I have an IQ over 9000 so I could understand EEEVERYTHING the Doctor was saying about inter-dimension time travel!! /s /kms