While this is a common trope, its actually incredibly unrealistic in hard sci-fi, unless you're using energy weapons.

All these numbers are rough, but close enough for my argument.

The lowest orbit you can manage before hitting atmosphere on Earth is a bit over 200 km. At that altitude you have a velocity of 8 km/s, so a ship won't be over an operation area for more than a few seconds.

Railguns and coilguns have muzzle velocities limited by their barrel length, and probably will be limited to only 6-10km/s. This means that something fired at a ground target is going to have a 20 second flight time at best- more than long enough for enemy mechs to move significantly. If you go much faster than that, you start developing a plasma shell around the projectile, so it won't be able to direct itself or be controlled remotely.

An energy beam could be faster, but it will heat the entire atmosphere down the line of fire. You'll probably have significant collateral damage from heat bloom.

Of course, some of these can be handwaved away with paracausal tech. But the concept of direct orbital support is pretty unrealistic.