Here's a hasty chart showing the current records for teams facing "good" clubs (those with records over .500). Note that this year, this list could change dramatically from day to day; we have so many teams hovering around .500 that it's funny (that's "peculiar," not "ha ha."
The teams are sorted in descending order of success in the current season.
The records against .500+ teams in 2013 are shown side-by-side with this sort.
A few teams with sizable performance differences are highlighted.
Note that the A's are currently having a run of games against sub-.500 teams (or maybe that's teams they've pushed back under .500 from beating them). They've only got nine games against viable opposition. The next closest is Milwaukee, which might possibly explain their hot start this year...cold opponents.
The Rays, who usually hold their own against good opposing squads, are really struggling this year. The Astros, having another bad year, are actually improving from a mind-boggling .191 WPCT against good teams in 2013. (That WPCT is the sixth worst such WPCT since 1901, ranking 2357th out of a total of 2362 team-seasons...).