it is the way it is. And it is not really an insult. When we describe things to people we always make comparisons so they can get a loose idea of understanding. Even if it is only a partial comparison.
Thanos is his own thing. But he could be the main villain for a bunch of movie goers. Or at least has the potential to be. Which is kind of cool.
Even in the comics it took a fair bit for him to catch on. That said, I think when he was used more sparingly at the start of things it was better. It has been more annoying to have him show up in every other title. Or at least that is how it seems.
I think that such comparisons lessen the power of the second, and make it seem as if it can't stand as tall on its own. Forever keeping it in the shadows of the first.
AS Lisa Simpsons once said, whenever you say something is the something of somewhere, its nothing.