By mobility you mean what? Speed, steering ability to move up the hill. The APC has a 120mm cannon and it is wheeled of course that extra weight is going to affect its movement.

I was more specifically referring to acceleration and inclines. Yes it is heavy for a wheeled vehicle, but it shouldn't take nearly as long as a 65-ton MBT to traverse a paved incline, far slower than a wheeled vehicle, and it should be able to at least traverse the Stratis steep incline without stopping and having a tea break partway through.
The following comparisons were all performed using fast move (shift+w)
Comparison of the Marshall, Slammer, and Rhino on the Stratis test track:
https://youtu.be/hR4EFrMpRsw

Comparison climbing the hill on the Tanoa test track (time accelerated):
https://youtu.be/o0j5SdriVhs

Comparison climbing a paved hill on Malden (time accelerated):
https://youtu.be/1eb0_9z_81E

I think the speedometers speak for themselves here. May I also point out that the Rhino went last and had more of a run up to gain speed. When you start comparing vehicles similar in role to the Marshall and Rhino, IRL, you will likely find that they tend to have power/weight ratios not far off. If the Rhino is a purpose-built vehicle, then would it not have increased engine power to keep pace with the motorized forces its supposed to support?

To get a little scientific, I don't have stats on the AMV 40 which that Marshall is based on, but since it's roughly analogous to the LAV-25, we'll use its power to weight ratio of 19.5 hp/ton (~15 kW/t). A purpose built 8x8 tank destroyer like the B1 Centauro? 19.35 hp/ton. Type 16 MCV, 21.9 hp/ton. ERC-90 Saigae, 18.7 hp/ton. AMX-10 RC, 18.7 hp/ton as well.

The Rooikat, which the Rhino appears to be based on, 19.24 hp/t. Yes the Rhino has a much larger and heavier gun, but also a more low-profile and lighter turret, and even then a builder would at least try to increase power and torque to gain some of that mobility back. These are all purpose-built vehicles similar to the Rhino, not converted from another vehicle with no powertrain modification like the Stryker MGS.

Modern purpose-built wheeled TDs are designed to maneuver with and support motorized forces. If it cannot do so, then what you have is a lightly armored tank with worse off-road performance. I'm not saying we need to turn it into a racecar, but if I were in charge of acquisitions for NATO, I would send the Rhino back in its current state to whoever built it and demand a more powerful engine. If it has the same level of mobility as the heaviest and slowest, MBT in Arma, then what's the point of it in the first place?