It’s true, house martins can be messy and leave unsightly areas of droppings around their nests. This could be a nuisance if the nest is close to a window.
The easiest thing to do is to fix a shelf about 250mm wide about two metres below the nest to catch droppings. Make sure it is at least that far below the nest to allow the house martins plenty of space to still fly up to their nest.
Use keyhole brackets so you can remove the shelf for cleaning once the house martins have left in autumn.