。
She is all for going shopping.
Literal
She [topic-は] shopping [to-に] go [nominalizer-こと] [to-に] big-agreement is.
大賛成 is the intensified form of 賛成 ('agreement') — 大 ('big') prefixed for 'completely in favor.' 賛成する takes に for what one agrees to. Here the agreed-to thing is the act of going shopping, nominalized with こと.