Each division has decent fighters up to about the top five, but usually after that (and sadly sometimes before as in the case of the Flyweight division as there's not much talent there beyond the Champ) there is a significant drop-off of female fighters who shouldn't really be in MMA let along fighting in the "prestigious" UFC.
It wouldn't matter so much if there were stars to carry these weak division, as has already been said Rousey drew huge numbers & media attention facing very weak opposition, until Holm & Nunes. But no-one else has got anywhere close to that level of interest. Nunes should be a megastar, a lesbian, two-weight champion who is killing all opposition but hasn't really crossed over into the mainstream. Best option is Zhang, an exciting fighter, Asian heritage & has by far the best run of possible contenders of all the female division.
The UFC needs to cherry-pick the best fighters out of Invicta & swap them with promising fighters who are in the UFC but really shouldn't be yet, then use Invicta as a development company where fighters can gain the experience & skills ready for the Octagon rather than throwing them in the UFC before they are ready & making a mockery of the fighter, the division & WMMA in general.
The sad part is that when done well with the top talented fighters WMMA can be genuinely exciting & pull the fans in. Even those who post on here just to dismiss WMMA can't honestly say they weren't looking forward to Nunes vs. Cyborg or that the fight wasn't exciting.
I don't think being a "lesbian" is some huge + factor for star-power outside the minds of social justice folk, niche groups, and politically driven interests.
Being serious here. Yes ~50% of the world is women, but what percentage of those women watch the UFC? Pay for PPVs? Tune in regularly / are hardcores? I doubt very many at all. How many women watch the NFL? I bet it's less for the UFC/MMA.
What I am saying is the reality of the situation is: Straight + attractive female = $$$. That sells way more than someone like Nunes, who isn't "clasically" attractive and is a more masculine lesbian sexually and aesthetically. Nunes is a great fighter, obviously the GOAT as of right now in WMMA and I'm not saying she isn't a draw either, but specifically on that one point...being lesbian.
I'm just not sure that in the real-world "being lesbian" is some trophy. Yes it's a talking point for a political slant or being inclusive or "SJW" in some way, but I don't think it sells fights or gets views at all. Pennington and Torres are lesbians too, I mean fuck, half the women's roster is probably bi/lesbian - Andrade, Gadelha, etc.
I agree with most of what you're saying and I'm not trying to myopically zoom in on a single point. I just don't think Nunes is some awesome star. here's why:
She can't speak english well, she simply can't. Yes she can talk unlike Anderson Silva for example, but it's broken english. She isn't classically attractive or pretty, and wears masculine clothing, is a lesbian - aka the appeal for men sexually isn't there. She isn't outlandish or a showman, she is respectful, she doesn't do exciting things outside the cage.
Simple as that. She's great and exciting inside the cage, and that's why she has some drawing power even for a WMMA fighter. But if she was all these things and never beat the shit out of Rousey she would have far less star power than she currently has, Cyborg and Holm factored in too. If she was a lesbian who was a great fighter and exciting in the cage but never crossed paths with Rousey, less so Cyborg, and even less so Holm, and never had the belt let's say, she would be a nobody. Even with the belt minus Rousey, she would basically be a nobody.