सुभद्रादय परिवेष्ट्रारोऽस्येति सुयामुनः; गोपवेषधरा यामुनाः परिवेष्ट्रारः पद्मासनादयः शोभना अस्येति वा सुयामुनः ॥
In plain words
Subhadrā and the others attend upon him — and so he is called Suyāmuna. Or take it this way: his attendants, the Yāmunas in their cowherd dress — Padmāsana and the rest of them — are splendid, and that splendour is the su in his name. So he is Suyāmuna.
Close to the Sanskrit
Subhadrā and the rest are the attendants of him — thus, Suyāmuna. Or: the Yāmunas wearing cowherd garb, the attendants, Padmāsana and the rest, are excellent, of him — thus, Suyāmuna.