Wingbone

You are asking two differant questions. Can they digest corn? Yes... and no. Since they are ruminants, they require bacteria in their rumen to aid in digestion. In an area where they have access to corn year 'round, their rumens are acclamated to corn and they can digest it. In an area where they mostly feed on native vegetation, their rumens are acclamated to leafy vegetation in the summer, slowlyshifting toward woody feed in the winter. It is these deer that, fed corn or hay, can't digest it. The bacteria aren't set up to do it.
The second question: Is an all, or mostly corn diet good for them?
No. Corn contains lots of energy they need, but few of the neccessary nutrients they need to be healthy.