Wait / run around dodging till she lands, then clear other enemies.

Generally its possible to clear the area before the fight and not be annoyed by mobs but some spawns can be persistent and will respawn them given the chance, so you have to plan where to fight accordingly. Mostly just avoid fighting near golem spawns and eggs.

Considering Moder is mechanically easier than most other bosses additional mobs do balance things out a bit. Kind of like with Bonemass and Elder having helpers. The only difference is that Moder fight difficulty is more emergent property of terrain rather than designed into boss itself.

Dont worry, next boss is gonna be even harder than Moder with mobs around