The Mariners just traded for Kendrys Morales, because their offense stinks, and he is one of the few available hitters they could acquire at the moment. Of course, Morales has been awful himself since joining the Twins, which may or may not be related to the fact that he sat out the first two months of the season. However, you will see it noted that Morales is "coming around", and that he just finished a 12 game hitting streak, so perhaps the rust is wearing off.
Let’s take note of what he actually did during that streak, though. During those 12 games, he hit .292, which is good. He drew two walks, which is bad. He didn’t hit any home runs, which is also bad, given that the only thing he’s paid to do is hit. His overall line during the streak? .292/.314/.417, good for a 100 wRC+. In other words, during Morales’ "hot streak", he was exactly a league average hitter.
And when you’re a Designated Hitter who also is one of the game’s very worst worst baserunners, hitting at a league average rate makes you pretty much useless. The Mariners are going need Morales to keep "coming around", because even during his 12 game hitting streak, he wasn’t any good.