Mechanically, the Cloak of Displacement only gives a creature disadvantage on attack rolls against you.

Since Magic Missile doesn't require an attack roll be made against, the cloak has no effect on them and the missiles still hit.

Mechanically speaking, the Cloak of Displacement only gives a creature disadvantage on attack rolls against you. It makes it harder to hit you - in the case of things that require attack rolls - but it doesn't make you any harder to see or any less of a target.

Since Magic Missile doesn't require an attack roll be made against you and targets a creature that can be seen, the cloak has no effect on them and the missiles still hit.