What are the rationale behind this anhedral high mounted wing configuration? Is that usual for very large carge aircraft?
Anhedral wings will induce roll instability and improve roll maneuverability.
In a large/heavy airplane with a high-wing configuration there is usually excess roll stability, so this type of wings can be pretty common.
Both the high wing configuration and wing sweep contribute a negative sideslip-induced rolling moment, and anhedral is necessary to limit this moment. If sideslip causes strong rolling, the aircraft will have a tendency to dutch roll.
Yes, this is common in heavy cargo haulers.
As you probably are aware, a dihedral wing configuration provides roll stability. Roll the plane, and it will naturally roll back to level. A center of mass well below the center of lift provides the same effect. Put a lot of weight down below the center of lift, and you'll get the roll-back effect again.
While that's nifty, you can get too much of it. Too much stability makes the plane very hard to turn or otherwise control. You can also get a sort of aerodynamic pendulum-like effect called a "Dutch roll" as the "roll back" action over-corrects and causes a swing back the other way, and back and forth with a combination yaw and roll that is surprisingly effective at inducing air-sickness. Putting an anhedrial angle on the wing counteracts the "too-much-stability" problem created by the weight distribution, making the plane easier to control again and reducing unwanted oscillations.
Why not just make the wings lower? That may solve the stability problem, but it would create other problems as well. Just how low would the wings have to be? Would that design affect safety for ground operations or unpaved runways? And what about cargo -- how well does your new design perform in empty versus loaded configurations? How tall would the landing gear have to be to make such a design safe... and how does that affect loading and unloading?
Some aircraft, even the comically large Airbus A300-600 Super Transport "Beluga", opt for the low-wing solution, which invariably leads to a dihedral.
Airbus A300-600ST - photo credit: Airbus
But in certain cases, the high-wing plus pronounced anhedrial design has won out for some heavy cargo hauling designs, and is particularly popular with military where they may have to operate out of dirt runways and need to get the wings up away from the ground, and where short landing gear is desirable.
CG Galaxy at takeoff -- Photo Credit: USAF
The anhedral is rather exaggerated in the top photo.
If you look for pictures of the Mriya in flight the wings are more-or-less level. On the ground, fully fueled, the wings with 3 engines each are heavy and will bend down a very noticeable amount. The B-52 has a similar issue, to the point where it has outrigger wheels near the end of the wings to keep them from scraping the pavement.
Both the high wing location and sweepback increase roll stability. Too much roll stability not only reduces maneuverability but is likely to lead to Dutch roll, so anhedral is added to compensate.
Airplanes with high but unswept wings generally have neither anhedral nor dihedral.
Edit: Here's a discussion of stability, dihedral and Dutch roll, albeit in the context of RC gliders.
It is not unusual for a dedicated cargo plane design to have a high mounted wing design. It allows for the fuselage of the plane to sit lower on the tarmac while maintaining the ground clearance for the engines and wingtips.
It also makes it easier to allow the cargo door to be opened in flight without compromising the structural integrity because the main strength of the fuselage comes from the spine in the top where the wings and tail attach to.
Looking at the list of military transport aircraft, most use the same wing design. The only exceptions are those based on a civilian aircraft.
The wing is so long that it will bend significantly when lifting forces are applied. Therefore an initial anhedral angle is present to correct for some of this bending, as without it the resulting dihedral in high lift situations would be excessive and would cause a lack of roll control.
It is interesting that the some of the lightest aircraft in the world-- flex-wing hang gliders and powered "trikes" -- have a similar anhedral configuration to the massive An-225. The reason in the case of the hang glider is to maintain good roll response even in the presence of sideslip due to adverse yaw. I.e. to keep the wing's overall "effective dihedral"-- the roll torque generated by sideslip-- close to zero in the part of the flight envelope where handling is most critical, despite the fact that the swept-wing or delta-wing geometry contributes a dihedral-like roll torque in the presence of sideslip.
In a modern hang glider, the pilot is freely hanging by a strap connected near the CG, so when he is not exerting a force with his muscles his weight effectively acts at the CG, so there is neither the "pendulum effect" (CG well below center of drag of wing) nor the aerodynamic interference between the fuselage and wing (see https://www.av8n.com/how/htm/roll.html#sec-other-slip-roll ) which contribute to shift the "effective dihedral" in the positive direction in a high-winged aircraft like the An 225.
With a modern cargo jet adverse yaw may be well-controlled by other means, but too much "effective dihedral" (slip-roll coupling) due to sweep, low CG, and high wing placement still could create unpleasant handling in strong crosswinds, and could contribute to Dutch Roll oscillations. There could also be adverse consequences if the pilot failed to prevent the aircraft from yawing sideways to the airflow upon loss of power from one or more engines. (See related comment within this answer to another question -- https://aviation.stackexchange.com/a/56481/34686 )
When thinking about roll stability, it is important to remember that slip-roll coupling -- i.e. positive "effective dihedral" -- is a key part of roll stability and is the reason that a high-wing plane will tend to be more stable (or less unstable), in terms of roll stability or spiral stability, than a low-wing plane designed with the exact same physical wing shape including dihedral angle. The high-wing will also be easier to maneuver through banked turns using the rudder alone, via slip-roll coupling. (A fun exercise to try -- ask yourself "could I land this plane if the control yoke fell off?) But the high-wing plane will be less pleasant to handle in strong gusty crosswinds, and will suffer more of a roll rate penalty if the pilot doesn't use the rudder properly and allows the aircraft to adverse-yaw and sideslip while entering a turn.