My mental health has improved since having children. Before my first I was very seriously burning out at work, and had suffered with on/off fluctuating depression from my teens - I just didn't really recognise it, it was completely my normal.
Firstly my children added a wonderful dimension to my life, keeping busy, actually having to do fun things etc! My family life, of which my children are the greatest part, is a real refuge for me. For the most part I really enjoy being with my children.
Secondly, they motivated / triggered me to seriously reflect on my mental health, relationship with my parents, childhood. Having children caused me to unlock and address a lot of stuff. I used to indulge in my misery a bit, suicidal thoughts were kind of a comfort blanket at times. Obviously that isn't an option now and I had to move out of my comfort zone and really address what was going on with me.
Last thing is I've never felt an imposter being their mum. Its a role in my life that I feel I can perform well (enough) whilst being totally myself. Not without constant effort of course but I find parenting very rewarding and validating. It makes the angst about my other roles (at work, within my family, within friendships) just... Less important?
However, I do think a lot of mental health improves as people get on into their 30s anyway. And I have also had blips since the children were born, getting through some very difficult experiences. But the overall trajectory has been positive and my resilience has improved.