IMO, we did not sign Yoshida for his D, and expecting value from the defense would be a mistake.
That being said, for his salary, it's not unreasonable to expect better offense than he has given us, so far. He has been a bit inconsistent, so at various times in the season, he has looked better or worse, so maybe over the full season, we may think differently.
I'm not ready to call the signing a failure or even a bad one, but I do feel like it would not be right to say it was a "good signing" at that cost.