Why is the front section of the Su-30 not aligned with the longitudinal axis of the body?
This is not necessarily the best way to look at this airframe;  it is better observed in a 3-view to understand this.



Viewed from the side and top, the main fuselage does not curve down but blends into pancake like lifting body which mates with the wings and empennage of the aircraft; the engines then are slung under the fuselage in a linear pylon which blends into the fuselage at the tail the design is beneficial for several reasons.


Fuel capacity: The Flanker family of aircraft all have very long legs and can carry a lot of gas due to this flattened mid section and makes a good solution for the need to carry a lot of fuel.
Aerodynamically clean: the blended wing/body design offers low drag as well as low wing loading, making the aircraft very maneuverable as a consequence of high drag during high alpha flight.
Stores carriage: blended wing/body offer a lot of ordnance hard points (12 to be exact) for carriage of weapons, fuel tanks, sensors, etc.


The forward action of the fuselage is based upon the the SU-27UB airframe with an aft crew station raised in order to better facilitate flight instruction in the airplane this required raising this section of the fuselage.  The nose and radome appear to droop down but are laid out that way to increase the real estate for a large radar antenna in the nose radome for increased detection ranges.

Like most East Block war machines, the SU-30 is brutally functional, refined only where needed and passed up for utility elsewhere.  While it may appear to 'curve' the fuselage is essentially straight and the engines aligned with the COG.