Katifāhā means “her two shoulders,” and afraʿat evokes rising or being lifted. Fī muʿālin muṣaʿʿad intensifies that vertical aspect: the shoulder region forms a notably raised line.
This is more precise than calling the camel simply tall. The description isolates the position and contour of the shoulders. Recognizing the dual form is grammatically important and also preserves the paired anatomy the poet wants the audience to picture.