I would state this a little differently.
The old adage is that a successful team should play about .500 against the good teams and beat up on the bad teams.
Part of the challenge in evaluating this depends on what exactly you consider a “good team”. Some times you see this based on teams over and under ..500, but I don’t think that’s the best way to do it because it lumps the average teams with the very best and very worst teams.
I took a look at how the Brewers have done against the 9 teams that are completely out of playoff contention (Mets, Tigers, Royals, A’s, Pirates, Cards, Nats, White Sox, and Rockies). The Brewers are 23-18 against those teams, which I think everyone agree is not “beating up on them”
Against all other teams (the “good” teams and the average teams still in playoff contention) the Brewers are a decent 39-36, but against the best teams (ones currently with better records than the Brewers) they are 15-19. Against the very best teams in the NL (Braves, Dodgers, Phillies, and Giants) the Brewers are 6-13, which is clearly not good.
That 10-3 record against the Reds is the main thing keeping the Brewers in the position they are in. It will be interesting to see what the Brewers pitching can do against the Dodgers and Rangers, two of the very best teams and offenses in MLB.
So, as a conclusion, I would say that the Brewers have done OK against the middle of the road teams, not very well against the best teams, and not as well as they should have done against the worst teams.