It's a bit off to ask people why they don't like Miranda. If its purpose as a sitcom is to be funny and you don't find it funny then that is all the justification required surely. I've watched a few episodes when at family's house and just not found it amusing. If I had to try and specify - the characters are ridiculously broadly drawn, I found it quite mawkish, and I didn't think the writing was up to much and could have come straight out of an inoffensive 70s sitcom.
I do think there is a strong reaction when something you don't like seems to be a bit of a sensation though. I saw there was some celebration of its ten year anniversary last year being broadcast from the albert hall or something, which annoyed me a bit until I realised that was an irrational feeling. It's like when people talk about how amazing OFAH was and I am genuinely perplexed