The actor got tapped by the network to be the guest prediction picker ahead of Saturday's slate of games, especially the big Iowa matchup later today between Iowa State and the University of Iowa -- with the latter being Ashton's alma mater.

As soon as AK touched down by jet and was revealed as the big surprise for the broadcast, he got torched by the college crowd that were standing by at Jack Trice Stadium.

They were chanting "Take a shower!" pretty damn loudly ... which the mics on set were picking up and broadcasting while Ashton was being intro'd. Doesn't seem like he heard it right then and there, but don't sweat ... the Cyclone fans had another go at it.

Not too long after, Ashton eventually plopped down on his chair for the pre-game panel ... where he was talking football and charities of his -- and there, too, the "Take a Shower!" chant picked up again. This time, it was way closer ... and Ashton probably heard.

He didn't flinch ... acting as if it didn't bother him. And, who knows ... maybe it doesn't. The guy already made fun of himself over the whole bathing fiasco earlier this summer.

ICYMI ... Ashton and Mila Kunis revealed back in July that they didn't really wash their kids daily, and only threw 'em in the shower as they saw fit, or if they saw noticeable dirt. It sparked a huge debate about kids and cleanliness -- but the Kutchers took it in stride.