Let's compare him to Bissouma;
In terms of scoring goals, Bissouma has scored 2 goals in 81 PL games, Propper 2 in 107.
In terms of making goals, Bissouma has 0 assists in 81 PL games, Propper has 7 in 107. Last season, Propper averaged 2 shot creating actions per game, this season Bissouma averages 1.62 shot creating actions per game.
In terms of giving away the ball a lot, on average Bissouma gives possession away once every 44 passes, Propper once every 43 passes.
Now, i'm not saying Propper is better than Bissouma but he's clearly not 'not very good'.
Bissouma is 12th in the league for interceptions and 2nd for tackles this season. I haven't checked Propper's stats from previous seasons but I'm certain they don't come close. Propper has been a good player for us and shouldn't be described as 'not very good', but your comparison to Bissouma feels incomplete.