I always thought this track was influenced by hashish or something potent like that.
I have a Smile bootleg that has the full version of "Our Prayer" - After the beautiful singing, they break down into stoned laughter. It's amazing they could sing so well while totally loaded. I haven't smoked anything in a decade, but I sure sucked as a musician when I did (I thought I was great, though).
I agree with Jack, it was probably just a common influence. Smiley Smile was recorded in mid-67 and Zappa had already done Doo-Wop type stuff.
I know Smiley Smile, Wild Honey, Friends are slagged-off by some, but I think they're great albums.