'Big Brother' -- Spencer's Mom SWEARS ... My Son's Not a Bigot!

"Big Brother" star Spencer Clawson is not a Hitler-loving bigot -- he's just saying terrible things so his racist friends in the house will like him ... so says Spencer's mother.

TMZ spoke with Donna Clawson ... who insists her son wasn't showing his true colors when "BB" cameras caught him praising Hitler and referring to a gay housemate as "Kermit the f**."

"Spencer really is a great guy," she tells us ... "This is not a guy that’s a bigot or a racist. And he’s not that way at home. He's a great guy."

So why would Spencer say such horrible things? Mama Clawson says her son is just trying to fit in with the racist, homophobic people in the house ... because it's the only way he can win the reality show.

"If you're in a group, you go along with the group talk and that is what is happening with him."

She adds, "This is a reality show. The purpose of the show is scheming behind each other's backs."