wRC+ isn't measuring a teams ability to get hits or baserunners, its capturing how well they do at creating runs relative to the rest of baseball. Over the last 30 days the Cubs have a 109 wRC+ with 100 being average. They are above average at creating runs against left handed pitchers.
It shouldn't be the only thing we look at, but if we're looking at one all encompassing stat, that's one of the better ones to capture performance. There are other stats we can look at that show the Cubs as more middle of the league vs LHP, but I don't really see anything that suggests that they are towards the bottom of the league in anything.
Could they stand to get better vs. lefties? Absolutely. Are there playoff caliber teams that have performed significantly worse against lefties? Yep, several