The scenes I found funny probably weren't meant to be funny.
Like when Tony's brother Frank left him a souvenir before he departed, which souvenir turned out to be his priest garb. Tony puts on the collar, looks in the mirror, then pulls up the draped portion into a "rope" and pretends to hang himself with it.
The bell-bottoms, platform shoes and short leather jackets the guys were wearing probably weren't meant to be funny, but in hindsight, looking at them today some 44 years later, they are a bit comical.

The scenes I found funny probably weren't meant to be funny.
They include:
The puerto rican woman getting knocked out cold in the gang fight at the pool hall.
Annette's gigantic bug eyes and the face she is making when they trick her that they jumped off the bridge.
Bobby C acting stupid with his 'look at me' and his stupid, fake laughing when he is on the bridge showing off for his friends. That scene went on way too long.
John Travolta happy to get his crappy job back at the hardware store.
Mother is the name for God on the lips & hearts of all children -Eric D. Raven

Double J: She ain't cum yet?
Tony: Since when do you care?
"I really wish Gia and Claire had became Tanner" - Honeybeefine

Tony's parents were even more ignorant, crass, and uncouth than he was. A lot of the laughs came from them, for sure, aside from his mother's outburst about the priest brother.

The scene that always makes me laugh outside 2001.
OK, it's happening.
It's happening, it's happening!
I'll be out in a minute.
Harder! I'm coming, I'm coming!
What was your name?

Annette: "YOU F-CKERS"!
"I really wish Gia and Claire had became Tanner" - Honeybeefine

If Tony was so concerned about his new shirt why didn't he eat dinner in his undershirt?
He was getting ready to leave for 2001, when his dad made him come down to dinner. Of course, he had a huge bib on to make sure he didn't get anything on his outfit.
"How about some gold plastic?" - Road House

If Tony was so concerned about his new shirt why didn't he eat dinner in his undershirt?

Basta means "enough". Then the nonna (grandma) said "mangia", which means eat. Did she say anything else?

This message has been deleted.

"I work on my hair, and he hits, he hits my hair".

Yes, I still laugh at that scene. It could be anyone's family at any time. Now a days, I think, "What John Travolta wouldn't give to have that gorgeous hair."