If you argue that then you must consider that Godzilla himself is mutated as well therefore making him a mutant. In fact half of the monster roaster would be considered mutants if you are considering that Mutuat definition is clear as day.
Godzilla, anguirus , Rodan, zilla , kumunga, and kamacarus are all mutated by nuclear radiation in some way.
Mutation under Webster dictionary
“Mutation :a relatively permanent change in hereditary material that involves either a change in chromosome structure or number (as in translocation, deletion, duplication, or polyploidy) or a change in the nucleotide sequence of a gene's codons (as in frameshift or missenseerrors) and that occurs either in germ cells or in somatic cells but with only those in germ cells being capable of perpetuation by sexual reproduction”
This leads us to Titanosaurs. Sad to say but he isn’t a mutant. His bio on Gojipedia or wikizilla do not state he was a mutation, just that he’s a prehistoric monster. The only mention he was a mutant is in Godzilla unleashed, however his unleashed profile lacks anything to indicate that he was mutated by anything, thus making his placement more akin to the earth defenders than mutants. Godzilla unleashed is not cannon to the main timeline and I would add it is part of the Atari timeline. But the info in his profile is a contradiction to his placement in a faction.