i do understand why he is stuck in the middle here - if she genuinely does dislike you and has told him not to bring you - and is likely to kick off on you if you go - as i suspect my fellas mum would at the moment - then i can understand why he feels its easier to just keep you separate.
at the end of the day, the events are his mums, not his, and its up to her who goes.
if i had a do, i would also not want someone there that i hated.
i do agree he should be more understanding and be trying to sort the problem out between you but equally, if he knows it wont happen, he cant be expected to just not see his mum or family or attend family occasions.
the issue in my case is with me and his mum - he cannot just 'fix', no matter how much he would like to - he is stuck in the middle and i would never expect him not to see him mum or go to family events because of me - and if the situation was reversed there is no way in hell i would not see my family etc because of him.
at the moment i would not want to go to any family do, because i would not want to get into a confrontation with her, but in future i am pretty sure it will start to irritate me if she doesn't calm down - as i will not be bullied etc.
its a tough situation for you, and i dont think its fair to blame your partner, he is just trying to appease both of you as he cannot 'fix' it
all you can do it speak to her and work it out
it could be that he uses his mum as a soundboard and has had a good old rant and moan about you to her on numerous occasions, out of frustration, just to let off steam, to get it out of his system etc and its kind of backfired as she now thinks you are awful.
that is exactly why i didnt bitch much about my fella to my family -as i didnt want them to hate him and have issues etc