As someone who grew up with porn in the 90's, I don't feel like the issue is whether or not the girls nowadays are "less attractive" than the girls from back then. I believe there are two major factors that contribute the differences between porn era's: age and quantity.
During the 90's and pre internet and pre gonzo days, you had far less women in porn and they generally ranged from ages mid-20's-mid 30's. Also there were a lot less companies that ran porn studios such as Vivid, Wicked, etc. and allowed for only the top quality of women to really shine: like Jenna Jameson, Tera Patrick, Serenity, Chasey Lain, etc.
Nowadays however, the problem is that damn near EVERY girl is in a porno. Look at any of the big sites like Brazzers or Bangbros, look at their catalog of women and you'll see literally hundreds of just regular random girls in at least one porn vid, all ranging from just turning 18 one-and-done girls to the established. And that's not even mentioning sites like GirlsDoPorn or ExploitedCollegeGirls or other sites that primarily deal with one-and-done girls. It's also a hell of a lot harder for a girl to truly stand out in this business because of the over abundance of women. The most popular women in porn today barely have a fraction of notoriety compared to the most popular women from the older era's. I'm not sure if the internet is responsible for this or if there was some kind of social shift somewhere, but porn is all a numbers game now. Get the most girls; get the newest girls. And more 18-19 year old girls are willing to do a quick porn vid or two just to buy a pair of shoes they want or pay this month's rent, as opposed to the far less amount of women from the 90's who made the decision to make porn their careers.