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.
The An-225 was initially developed to be the transport aircraft for the russian space shuttle, the Buran
Image from Wikipedia
This design criteria most notably led to the particular tail design, as the wake of the Buran would have made a classical single rudder/empennage design totally uncontrollable.
The requirement for the cargo compartment has probably led to the high-mounted wings (shorter gear legs, easiness of un/loading).
The predicted Buran wings interference has thus probably led to the anhedral part, but I cannot find any official source on this. Another possibility is that this is a by-product of structural limitations of such large high-mounted wings.
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.