[quote=Dictionary]Mutation
mjuːˈteɪʃ(ə)n
noun

[...]

2.
the changing of the structure of a gene, resulting in a variant form which may be transmitted to subsequent generations, caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes.[/quote]

The cosmic energy caused his genetics to alter - he was still growing in the womb at the time of exposure, so he gained his powers as a result of his changed DNA. That fits the definition of mutation, even if it was induced rather than natural.