I pretty much agree with everything you say here, and I hope that this is one of the things we'll see changing as more women are recognized for writing good SF. The "dead mom" trope is more or less the equivalent of the "fridged wife/daughter/girlfriend" trope -- a cheap and easy way to provide motivation for a protagonist, or to get an inconvenient relationship out of the way.

Old though they may be, you might want to check out the Telzey Amberdon stories by James Schmitz. Telzey's mom is very much alive, and while the mother/daughter relationship isn't centered, it's not absent either. I did a re-read of the whole Telzey ouvre a few years back when Baen reprinted all of them, and was very pleased to find that the Suck Fairy has largely left them alone.

I'd also like to mention that the "isolation" thing sometimes cuts both ways. As a woman who chose not to have children, I got sadly accustomed to losing friends during my late 20s and 30s as they married and had kids. And it wasn't just the lack of time, although that does play a large part; it was also the distinct feeling that they no longer wanted to hang out with me because I couldn't swap kid stories with them. A few of them I got back once the kids were school-age, but usually not.