I'm also coming at it from the opposite side.
Big hugs to you, that sounds really tough. I know it may not seem like it now, but things will hopefully get better in time. I never in a million years thought that I would call Charles's mom my friend. We did not get off to the best start, to say the least. Things got even worse when she moved him across the country without consulting Jason at all. I was so angry and hurt, and felt physically sick just hearing her voice.
But over the years we have developed a friendship. It took a long time, but we have always had the understanding between us that we all want what's best for Charles. That's the priority. It's not ideal and we don't always agree, but we've always been civil to each other and we never talk trash about each other in front of him. We're all grown-ups and we need to act like it. We've always tried to put a positive spin on our situation. He has four parents who love him, lots of grandparents who love him, and he gets at least three birthday parties every year, LOL. He has more than the average number of people in his cheering section (also more people to fuss at him when he screws up)
I will say, as a stepmom, I am incredibly grateful for the relationship I've developed with Charles's mom. She was always kind to me, when she didn't have to be. I was so lost coming into this new family, not knowing how to make a place for myself. I had no idea what to do or how to act or what my role was supposed to be. Nobody thinks when they're little, "I'm going to be a stepmother when I grow up". That's not something you can really prepare yourself for. But a little kindness goes a long way, on both sides.
Every family works things out in their own way. For us, what works is to have a group text message between Jason, Charles's mom, and I. That way everyone is in the loop. She and I do most of the communicating generally, but that's just because we're both pretty chatty. We have a lot of similar interests and Jason doesn't need to participate in the reading/makeup/mealprep portions of our conversations. We exchange pictures and texts several times a week with her and it's been that way for a few years now.
For us it actually does work well that she and I communicate a lot. But if that doesn't work in your family, maybe talk to you ex about it? Not everyone's personality is going to mesh well with everyone else's, and that's ok as long as everyone is civil. Hopefully you guys can work things out in a way that makes everyone feel better about the situation.