Not at all. My own mum kept in close contact with her ex DiL - they both came from the same place so on the rare occasions that they met up. they spent time gossiping!
She also kept in touch with her ex SiL .
When children are involved, be they GC or nieces and nephews, it is so much better to keep in touch - after all, when a relationship breaks down, it is more often what has happened between 2 consenting adults - rarely are the in laws involved.
Mind you, she didn't keep in contact with her ex SiL - but then, he didn't keep in touch with anyone either, least of all his own child!