『女の子も とても きにいり いつも もう それしか かぶりません。』
I believe もう adds an emphasis to それしか かぶりません.
Without it, it means "they wear only that". もう adds the sense of "it's done", "from now on", or "there's no turning back", etc.
1) (without もう) Girls like it so much they never wear anything else.
2) (with もう) Girls like it so much that (it's a done deal / it's a foregone conclusion) they'll never wear anything else.