From the Douglas DC9 manual:
"The nacelles have been designed to enclose the JT8D engine with a shape that will allow the highest installed performance ( thrust minus drag ) possible at all flight conditions. The nacelle nose cowling is nonsymmetrical to keep the drag to a minimum at the high mach number cruising conditions.
The nonsymmetry is required to maintain an extremely flat nacelle line adjacent to the fuselage, such that the nacelle supervelocity increments, when added to those from the pylon and fuselage, do not lead to super-critical velocities with resultant shockwave losses.
The nacelle axis and pylon have been set at 3 degrees angle of attack relative to the fuselage to align them with the local flow conditions prevalent at cruising conditions. This is necessary to eliminate the interference and high induced drag that they would otherwise be subjected to. The nozzle is then canted upward relative to the engine axis, thus eliminating the pitching effects due to thrust."