I would have to disagree about Dickinson being rubbish. You have to bear in mind that she was a 19th century poet and nature was the theme of choice at the time. Her poetry is highly experimental and very innovative.
Anyway, she's not saying that nature is simple - quite the opposite in fact - she is describing its magnificence. When she talks about simplicity she is suggesting that what we take for granted as the basics of nature are still way beyond human understanding.
Like many of her contemporary 19th century poets (Wordsworth etc) she has a preocuppation with the inability (or not) of poetry to describe nature.