It isn't ineffective as such. The problem is that nobody actually knows how the mechs work. The aerotech even less so. It just happens to be the case that:

1. The automated mech production facilities did better during the galactic nuclear holocaust than the aerotech.

2. A lot of snobbery about mechs being the proper way to fight wars.

Despite this aerotech can do a lot of damage to mechs but a key point is the mech design is modular and can be repaired relatively simply in the field. Attrition losses among mech forces are rarer. When an aerofighter goes down it stays down. So they are rarer and harder to replace. They also can't go to all the places a mech can. Mechs have had fights on the bottom of the ocean and in jungle arenas.

Later in the history the aerotech takes on a larger role. In fact it was the combined arms exercises that stopped the clans.

Also you overstate the relevance of air power. If anything WW2 proved that the belief that air power would rule all was overstated. Since then there hasn't been many engagements of air against ground forces but air power was practically irrelevant in both Vietnam and Korea. Consistently air power proves nearly useless against infantry and does slightly better against armour.