I did my undergrad thesis on fictional characters/objects and truth value so I might be able to help out. It depends on your view of fictional objects.
If you just take a classical logic view of fictional objects (none exist), then the sentence is vacuously true simply because there are no fictional objects. The "x" in "every x" quantifies only over existent objects, since according to this view of logic there are only existent objects in the domain of quantification that "x" represents. Looking at the truth value of the material conditional, then whenever the antecedent is false the conditional is true. So the statement "x is a unicorn" is always false since there is no existent object that is a unicorn, and the statement is always true.
On the Meinongian view, in which there are nonexistent objects for every single set of properties (for example, an object corresponding to the set {unicorn, ugly} exists simply by virtue of the properties existing, so does the set {square, circle} and {square, circle, unicorn, ugly} and so on), the sentence would be false.
On the possibilist view in which fictional statements are true according to a set of possible worlds in which the stories take place, this sentence would be dealt with in the same way as the classical logic view. They assume that an intensional operator is put in front of the sentence "all unicorns are beautiful" and this intensional operator rates the truth value of the sentence according to the world in which the fictional story takes place. But there is no such story in this context, we're merely analyzing the truth value of "all unicorns are beautiful." So it would be vacuously true.
Fictional characters are a huge problem for classical formal semantics, because they just lead to unintuitive results. According to formal semantics, all unicorns are beautiful is vacuously true. But intuitively this is false.
A previous answer stated the following:
A unicorn does not occur in the world; but in a fictional world; and in these fictional worlds things are described as beautiful or ugly ie they are the properties of fictional objects.
According to these views in which there is an intensional operator in front of this sentence, the intensional operator is determined by the context. In this context, there is no intensional operator because we're not talking about any particular story! So this sentence turns out to be vacuously true even if we take the possible world semantics view.