No. Quite the contrary. Consider mandatory sentencing for a given crime that a legislature might enact -- that's a clear violation of the separation of powers between the Legislative and Judicial branches of government. In this case, there's no discretion by the Judiciary, since its sentencing decisions have been codified into law by the legislature.
At first blush it appears just to have sentencing guidelines, that those who are found guilty of a specific crime should be punished similarly. However, the power of the Judiciary should be that it assesses as much information relative to the crime as possible, and have the punishment fit the crime. The crime of "Stealing Bread" might carry the same punishment, but the punishment should be different if a perpetrator steals one loaf to feed his or her children, as oopposed to a perpetrator who steals bread every day as part of a "bread ring" which then fences the goods and splits the profits.
Discretion is necessary for the Judiciary to function properly.