I dislike my stepdaughter's mother

My youngest stepdaughter has a mother that I struggle to get along with. I'll always be respectful towards her and civil, because not doing so would be traumatic for our daughter and it's not fair to drag her into that, but I feel like she's not interested in being civil. I tend to notice a lot of passive aggression when we have to speak and she seems to dislike me. I've also noticed she's not really a good person. It's difficult to get along with her but I know our kid comes first.

Sorry, just needed to get this off my chest and didn't know where else to go because I can't tell anyone I know.