In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). - she is wearing means she currently wears In French, this verbal form does not exist (directly translated “elle est portant” is incorrect). Therefore, you can translate either “elle porte” or “elle est en train de porter”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.
The English was "she is (only) wearing" which is a continuous case and your question was about the grammar of the sentence. I therefore assumed you would be interested in knowing about the construction of the verbal form. However, if I taught you something, that was worth the error!