@aeroplume72 if i recall i believe mimikyu's ability uses the pikachu costume to protect it from a certain amount of damage before it breaks and needs repair.
so in simple terms mimikyu's pikachu disguise absorbed the beam.
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@aeroplume72 @Inuhentai & @deamon-flea got good speculation, but actual reason is that Mimikyu is already THICC if you look up at artist's previous comic.