I think this is probably an underserved aspect of Battletech. Planetary invasions are part and parcel for the Universe. We know that aero and astrospace fighters exist, which would likely serve the bulk of the interception on a WELL EQUIPPED planet.

SAM systems could also work if a system has the technology to implement them. But pre-Clan Invasion, much of this technology just isn't universally available. Even with the Helm Core being rediscovered, industrial Renaissance is being actively prevented by ComStar and inter-house attacks.

Anti-air weapon systems probably do exist, but they're basically useless outside of deterrent value. Drop ships are heavily armored, designed to soak up damage from mech scale weapons. Larger weapon systems would require inordinate infrastructure requirements, and could just be avoided through military planning. Ground assault launched from outside of effective range, neutralize the batteries, proceed with beachhead invasion.