The book is so chewed up that it's difficult to pin it down.  Mayhap it's a satire (in the modern sense of the word).  But unfortunately (or rather fortunately) we're so far removed from Nero's Rome that we have no knowledge of what's being parodied and so don't see the humor.  Was Trimalchio supposed to be a parody of Nero himself?  That would be sailing mighty close to the wind.

But perhaps the satire is timeless.  For we all have seen far too much of people very like Trimalchio.

Maybe it's a moral tale, telling us to avoid ostentation and to not think about sex every minute of the day.