Any sentence or fragment which is styled should apply the same style to its punctuation. In this case, the exclamation mark is part of the sentence being styled, so it should carry the same style as any alphabetic characters or punctuation within the sentence:
The brown fox (trailing the chicken's feathers) jumped over the hill!
A contrasting situation where you shouldn't style the punctuation characters is where they are not part of the sentence/fragment, for example brackets or quotes surrounding a styled part:
She typed "The quick brown fox (trailing the chicken's feathers) jumped over the hill!" very quickly.
In that case, the quotes are not part of the sentence/fragment I am emphasizing, so they aren't bolded.