I think YANBU - you'd think (hope?) most people would have done more than waved, they would have said "nice to see you again, sorry for your loss, how are you feeling?" - not just carried on, especially when the convo was about something as trivial as music.
Different story if one of them had been in the middle of a deep heart to heart about their own tragedy - of course it would have been harder to switch attention to you - but music? Nah.
Having said that, of course your feelings are more raw now because it's YOU that's still flailing under the grief of having lost your mother. For them, it doesn't personally affect them.
When my nan died (and she was my favourite relative), I went back to work after a couple of weeks (I'd been away for other reasons) and was still sad - but one of the other staff was (I felt) SO rude and callous to me, asking why I looked so miserable. To her, losing a grandmother was momentary sadness, not deep grief. To me, it was deep grief - but she couldn't understand it at all. OF course with a mother it's more obvious that you'd still be upset - but your friends have moved past their sadness-for-you, I'd guess.
This is why people usually fall away after a funeral - very supportive prior to it, but for them, the funeral = "all done, get on with life now", which of course isn't AT ALL how it feels to the bereaved person.
I hope that you can reconnect with your friends, maybe on a one-to-one basis initially, and deal with this. I don't think they're bitches, necessarily.