Screech's mom was also shown in the series.

Slaters mom was in prison most of the series because she was busted at LAX as a coke mule.

I don't remember seeing her mom, but I'll take your word for it. I haven't watched the movie in a while.
Using the word 'cuck' is equivalent to wearing Ed Hardy apparel in public.

True. Unless you count Wedding in Las Vegas, where we saw both of Kelly's parents. And a couple little brothers? Only her dad was seen in the show.

I think Zack and Lisa are the only main characters that we've seen both parents for.
Using the word 'cuck' is equivalent to wearing Ed Hardy apparel in public.

She's mentioned in the episode where Zack has a crush on the school nurse. Slater is trying to get out of getting a flu shot at school, so he asks the girls to write a note saying he already had a flu shot from his family doctor, and sign it "Mrs. Loretta Slater."
So apparently he did have a mom, she was just never shown.

No but in The Fight episode, which was the first day of the gangs senior year, Zack hired a middle aged Latina woman named Martha to PRETEND to be his mom and embarrass him on a date it was pretty hilarious
But that was the closest they ever came to showing Slatets mom

I don't think so either.

I don't think so