btfly I am so sorry you had this negative experience. And sorry you have had some well crap responses here! (as well as some good ones).
It may be that some are racist or don't like people with difference accents. You do get such people in all walks of life and in all places in the country. So you are not wrong to consider this may be the case. But generally I would say it is not.
Do you want people to just say hi or are you hoping to make friends?
In my experience by the time the children are at school it is mostly the kids who make friends, and we make friends with the friends' parents! Before that it was the other way round, I made friends with people with kids and their kids were my kids' friends!
As to why those mums were not friendly. It may well be that a lot of the mums are busy, wrapped up in their own things and not aware of others who could use a bit of human kindness.
I found with my daughter is was quite easy to make friends as we were all new mums together. All had similar aged babies and it progressed from toddler groups to pre-school and to school gates.
Our son is adopted and came to us at three, nearly four. So the whole mum and baby thing was not there and the todder group was not so easy. But I still took him at three and had the good luck to make friends with one of those mums who does everything, is into everything and gets people together. Without even realizing it (I am sure she would say she had not! but... ) she took me under her wing.
She introduced me to new people and invited me over for coffee. I invited her back and now we are good friends. All from meeting at a toddler group and ending up with our sons in the same class. The friendship has grown because we have made time to really listen to each other, and that cannot happen at the school gate. That takes more time.
In our school we are near a park and often, on a Friday, a lot of kids and mums gravitate there, or the pub garden for lunch at the end of term etc.
This all takes time so I would just say be warm and smiley, if you catch someone's eye venture a comment, the weather is a very good topic, it is usually doing something noteworthy. If they ignore you or blank you, just brush it off. If they reply, just keep smiling and if the conversations continues, so be it.
See if you child has any particular friends and would like to go to the park with them, and their mum, or would like to invite them back to play, with the mum coming back for coffee.
It is part of a much wider thing really, at the 'higher' level you may decide to start volunteering at the school, if you have time, or helping to fundraise, all these things help you to get to know other mums, and dads, and sometimes meet their friends and family. Some won't want this, it sounds exhausting to them. As an extrovert, I need it!
But start small. Start with a few well chosen words. Compliments sometimes work, a nice coat or a new hair style or their child bringing out a really good painting etc. But always make sure it is a genuine compliment about something you like or it will sound false.
Good luck. 