Non-coding DNA as a protection to deleterious mutations
You seem to be under the impression that there are a set number mutations per cell, which is not true.  The number of mutations is roughly proportional to the number of bases  — this is because mutations typically happen as a result of errors during DNA replication .1  Consequently, more DNA will not protect the coding portions of the genome from mutation.

The reason why mutations appear to be suppressed in coding regions is due to selection.2



References:
1: Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., &amp; Darnell, J. (2000). Molecular cell biology 4th edition. National Center for Biotechnology Information, Bookshelf.

2: Griffiths, A.J.F., Gelbart, W.M., Miller, J.H. and Lewontin, R.C. (1990) The Molecular Basis of Mutation. In: Modern Genetic Analysis. W.H. Freeman and Company, New York.