Well, the grenade does aim towards the angle between where it is spawned and the mouse, so that part looks like it works okay.
Do you mean you want it to have more of an upward angle (eg: lob it in the air then it falls?), if so then you might want to base the throw strength based on the distance between the grenade and mouse as well: distance(grenade.x, grenade.y, mouse.x, mouse.y) * POWER
Then lower the value of power.