That's a massive assumption mate and I disagree completely. Despite the reputation he's somehow managed to get from Liverpool fans for being a great shot stopper, he's actually a very bad one, consistently ranking near the bottom of the league in whatever metric you care to look at.
I do think he's the better goalkeeper of the two (although that says more about Karius than it does Mignolet), but Karius's distribution is better, which may help us break down shit teams, and I think the defence looks a little more solid in front of Karius as well.