For some reason my 18yr old niece said the same. She went with her friends last night and told us it was funny. Not sure how to take that as i did grow up watching the original mini series.
I find, girls with groups of friends laugh alot during scary or shocking moments, it's a tactic used to disconnect oneself from the fear, it's too terrifying to be absorbed so they quickly try to trivialise what they see to break the mood.
It happens with my family members and it annoys me to no end, because they spoil it for the rest of us who are happy to be absorbed and get lost in the fear.
The themes and overall plot is quite serious if you allow the movie to dictate the mood. Unfortunately when I watched IT we had screaming girls in the back being obnoxious and laughing straight after scares, outside of that we all found it scary & tense but were frustrated with people trying to sabotage the mood because they were terrified.