Most planetary defenders do not attempt to engage invading or raiding forces until they are in or near orbit. 'Mech units are assigned their air cover they have in order to repel attackers or take care of defenders when they go on the offensive.
That said, mercenary units are (usually, there are a handful of exceptions) short on ASF elements. Since the Great Houses have always made use of mercenary formations as sacrificial goats so their regular military does not suffer as badly, this probably provides defending forces with additional incentive to go after the mercenaries when they deploy with regular forces- easier prey coming in, and if you can get them before they land you can go after the real game.
To provide a more concrete answer, though, we need some more details; what is the defending force and intended landing site like? If the defenders are a bunch of infantry supported by a handful of tanks and all the enemy ASF have been taken care of, so long as the intended LZ is not in a location heavily mined by the defenders and surrounded on all sides with hidden positions, all should be good.
The larger and better-equipped the defenders are, the more problematic it becomes.
It should also be noted that what most captains want to avoid is having to put down under fire in order to disgorge troops. One of the advantages of 'Mechs is that they are readily deployed via combat drop, either from space inside a coccoon or inside atmosphere with a drop pack. Some of the best even deploy via hover-drop, where the DropShip halts just above the ground and unleashes its cargo; the Nova Cats are famed for their use of that maneuver.
Vehicle and infantry transports have a more difficult time of combat dropping their charges, and are the ones that are usually most susceptible to attacks while grounded. It's an even bigger problem given how many infantry and vehicle transports are Aerodynes.
Assault DropShips are relatively rare; they are held in reserve as defenders or deployed to particularly vital assaults. If the world is an important one or the unit they are going to one of high prestige, they may be able to garner such support. Mercenaries are even less likely to have one at their disposal.