This cracks me up. The past couple years under Klopp when we had an inconsistent defense everyone was frustrated that we were not able to keep a clean sheet, but when we do and are the best at it over half a season, suddenly they are overrated. Can't make this sh*t up. By the way, he also had the fewest goals allowed per 90 minutes since he took over 
As for your reason for the upturn, that is just brutal. Point out flaws and areas for improvement, sure, but to completely disregard someone's contribution to the team and say it was all due to someone else is as shitty as it gets. Even posters like Heimdall, who has a Karius pin doll, has admitted, even vaguely, that he did well once he took over as the #1. And why would you care who the goalie is when it's all really due to Van Dijk, Robertson and Arnold, anyways? 
Let's please not forget that context is key, particularly when evaluating player performance based on stats. For example, Karius and Mignolet played half of our PL fixtures each. However, Mignolet had to face 35% more shots on target (SoT) than Karius had to. That is not a small difference, and is a very good indicator of how much better our team defended in the latter part of the season before Karius has to step up to make a play as a goalie.
Another stat (that should be taken with a bigger pinch of salt, but still should be considered), is that Karius took 50% less high claims than Mignolet. One of the biggest worries for me with Karius is that he isn't a commanding goalkeeper who owns the box. Regardless of stats, our eyes were never convinced of this, from either him or Mignolet.
In the end, both keepers are capable of saves, and yeah Karius probably more so than Mignolet. But goalkeeping is so much more than that. It's about commanding an area and instilling confidence in the whole defensive unit with communication and actions. We certainly don't have a keeper who does that, or whom I personally think would grow into that.
The other thing is, when our keepers make their mistakes, it is often in big matches where we probably won't get another 10 shots on target against our opponent in order to have a good chance of equalising or regaining a lead. Against lesser teams we often can create the opportunities to get back into a game if a howler is made with enough time left in the match.
It's all about temperament. You see it in other sports as well. Just look at the NBA playoffs. James Harden had an MVP regular season with the Rockets, but when it counted most in the playoffs, he and his team went something like 1 from 29 from the 3-pt line (which was their bread and butter). Paul George from OKC just can't step up when the real pressure is on.
And then back to Real Madrid. Played pretty awful for most of the year, but had the temperament to focus and win when it counts most.
Anyway, if you can watch a game with both Karius and Lovren and not always feel like we're vulnerable when a ball is crossed in, then I'm envious, because it will rack my nerves for as long as they play for us hahaha.
