It isn't unreasonable to want something like that.
It is unreasonable to ignore that there many British variations, and many American variations, to shorten the word mother, the latter's version nearly all coming from Britain in the first place. Mom isn't really an Americanism, it's a British import to the US.
It's also without reason to get really emotionally disturbed by which version someone uses, but emotions are like that.
I know that there are regional differences but in books/articles it was always written 'mum' wasn't it? Mum was the British way of spelling it.
It became the standard expectation for how to spell it. Standardised British English used for writing is based on the ideas and speaking patterns of those who owned or could afford printing presses and making education books for some time, starting in the 15th century, largely set by the mid-to-late 18th with some minor changes since. There was a big push during that time period in a lot of places to standardise as much as possible and show those less fortunate how they 'should' be doing these things. It's always been a bit contentious in that, and there have always been people who ignored it for writing closer to how they talk and think.
In some forms of writing, that remains the expectation. In some fiction as well as casual writing, there is a lot more room for people to write how they talk.