Nikki is not better than Trish. Nikki has improved from complete garbage to above average, and appears to be putting in work to do so. I commend her for that. But that's all she is - above average. Trish improved from being just a model to being pretty damn good in the ring, far better than Nikki is today. Nikki is most certainly not the result of 10 years of evolution - she was just another model who tried to become a wrestler. She's done a better job of it than a lot of other models who decided to become wrestlers.
Trish also wasn't the first to make the women's division relevant.
Nikki is hot and appears to be a hardworker and she has a cool finisher. Compared to her sister and some of the other non-wrestlers the Divas division has been saddled with over the past few years, she's certainly not bad. But she's nowhere the best ever. She's not even top 10.
As far as the total package of being a wrestler, she's below (and this is off the top of my head): Trish, Lita, Victoria, Paige, AJ, Ivory, Mickie James, Madusa, Moolah, Wendi Richter, Chyna, Jaqueline, Sherri, Molly Holly. She's not as good a wrestler as Gail Kim, but I might say it'd be fair to consider her the better "champion." The flip side of that is that she's probably a better "wrestler" than Steph, but Steph was a better champion. She's also not as good as Charlotte, Sasha, Becky or Bayley, but only Charlotte has been a Divas champion out of those and it hasn't been a great reign so far.
Nikki is on par with Sable, Melina, and Beth Phoenix.