Yoshida has yet to play a game in Fenway, where he was excellent last year (and surely part of his appeal to us). At Fenway last year he hit .328/.391/.534 through August 1st, and .306/.360/.475 overall.
I'll admit I'm a little concerned. He's a fun character to have on the team, but the downside seems ominous and he's not an ideal roster fit. However, the sample we have for him this year comes from three pitchers' parks, with 40% of the games against a team with one of the best rotations in baseball.
There's also this: Yoshida has hit the ball 11 times with an exit velocity >95 mph so far this season. That is tied for 70th in MLB, with guys like Devers, Bogaerts, Harper, Altuve, Happ, Robert Jr., and others. Hitting the ball that hard generally gets it done, so I'm not especially concerned it's a matter of strength. It's the ground balls with him.