The Mishna says (Arachin 1:1):
חרש שוטה וקטן נידרין ונערכין אבל לא נודרין ולא מעריכין מפני שאין בהם דעת
A deaf-mute, an imbecile, and a minor are the object of a vow and are valuated, but neither vow to donate the assessment of a person nor take a vow of valuation, because they lack the presumed mental competence to make a commitment.
That second part (bolding mine) is obvious from logical reasoning.
Maybe "לא נודרין ולא מעריכין" is necessary to complete the list: if I'm told they can be the objects of vows of valuation/assessment, I should also be told they can't be the subject.
But why say "מפני שאין בהם דעת"? The only reason anything is different about a shoteh is because he lacks da'at. For the deaf person and katan, I might think of other problems, but here it's clear da'at is the reason, since they are listed with the shoteh.
Why does the Mishna need to mention this explicitly?