It was the biggest box office success for Warner Brothers in 2021, so not particularly mediocre..
That's a relative metric so it doesn't mean much. You have to look at the hard numbers:
production budget for Dune: 165 million + at least 50 million for marketing.
US box office so far: 41 million
International box office so far: 182 million
Keep in mind that the studio doesn't get all of that money. In the US the studios split the ticket sales with the movie theaters. It's usually about a 50/50 split or a 55/45 split. For international movie theater operators, they get a significantly higher cut of the ticket sales.
So yes, it's a mediocre box office performance.