Yes, a lot of advanced stats actually do take external factors like the quality of opposition into account. You should try studying them. Looking solely at one measure, like runs per game, to assess the overall quality of a team's offense is so very flawed I can't believe I'm having to debate it. And you can't just eliminate the American League from your analysis either, considering the whole inter-league play thing. But yes, if you look at things exactly the way you are, you can make the argument that the Brewers offense is below average.