@Wham with the Right Hand
Mercado was not unlucky last year.
A .300 BABIP is right at league average, and when you consider his average exit velocity was just 86.5 MPH (1 MPH below league average) it makes sense for him to have an average BABIP despite plus speed.
Given his low exit velocity, he's probably not going to hit many HR, so his 15 HR in 114 G is not something we should expect to repeat.
I'd encourage you to look at his xBA% on the website you cited.
xBA% determines how likely a batted ball is to become a hit based on similarly hit balls in the past. It accounts for launch angle, exit velocity, and as of Jan 2019, it accounts for a player's sprint speed on weakly hit grounders.
Mercado's xBA% last year was .262 last year which is just a hair lower than his actual BA% of .269. So, everything is pretty much right in line except for his XBH which were well above expectations based on how hard he hit the ball. His xSLG% is significantly lower than his actual SLG%.
I don't expect Mercado to be much of a power threat, nor do I think he needs to be one in order to be a productive everyday CF. Offensively, I'd like to see him improve his contact skills and draw a few more walks. Being a .270/.320/.440 (like last year) guy makes him one of the weaker hitters in any decent lineup. If he could get his OBP% up to around .340, even with a drop in SLG%, that makes a world of difference.
Defensively, his instincts and routes he takes need to improve. He's in just the 11th percentile in route efficiency. Now, a lot of really good CF rate poorly as route runners, but they make up for it with far superior bursts and great reaction times. So, yes, Mercado is in the 69th percentile for jumps among ALL OF, but among CF, he still has some ground to make up to be above average.