Breakdown of Iskreno, večeras mi nije do smijeha.
biti
to be (sam, si, je)
večeras
tonight
mi
to me
do
to, until, up to / in the mood for (as in "I'm not in the mood for")
Questions & Answers about Iskreno, večeras mi nije do smijeha.
Why is the verb nije used instead of nisam since the sentence is about how "I" feel?
This sentence uses an impersonal construction. Instead of saying "I am not...", Croatian literally says "To me it is not...". The person experiencing the feeling is expressed with the dative pronoun (mi), and because there is no true grammatical subject, the verb defaults to the 3rd person singular (nije).
How exactly does do smijeha express being "in the mood" for something?
Why does the clitic mi come after večeras instead of right after Iskreno?
Words separated by a comma, like the introductory Iskreno, sit outside the main clause and don't count towards clitic placement. večeras acts as the first word of the actual main clause, so the enclitic mi takes its mandatory second position right after it.
If I wanted to say I am in the mood for laughter, would I just drop the "ni"?
Exactly. You would replace the negative nije with the affirmative 3rd-person singular clitic je, resulting in: Iskreno, večeras mi je do smijeha.