Could've sworn there was a list of characters with the most posts on a porn site. Can't seem to find it now though. I remember Sonic being one of the top view.
Point is, Tifa isn't even close to being the most prominent hentai character. I think you just watch a lot of Tifa porn.
As mentioned, in Japan (and probably several other Asian countries), big ones are Fate and Touhou. Those 2 series have several characters who single-handedly have more porn than Tifa. Heck, some of the more popular vocaloids like Miku and Rin probably even beat Tifa. There is no doubt that she gets beat out by several characters.
If we consider the "western" countries, maybe we could put Tifa at the top.
https://rule34.xxx/index.php?page=toptagsI don't know how to really measure "prominence" of a porn character in the west. The above link shows Tifa as the character with most tagged posts. Though Isabelle (Animal Crossing) and Samus are not too far behind. And I personally think
D.va and 2B would be even more closely contested if they had been around as long as Tifa.
So pretty much, yes, Tifa could be possibly the most "prominent" hentai character if we ignore everyone that's more popular in Japan. Though it's not by a huge margin, and we would also be filtering out furry characters.