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.