Comparative of Inferiority

[yun]

An English study book I read says, "He is less stupid than I thought he was is better expressed by either He is not so stupid as I thought he was or He is cleverer than I thought he was.
The book does not explain why they are considered as better expressions.
Instead, it lists some more sentences of a kind and ask the readers to reword them as suggested because they are not good ones.
For example,
My mother is less old than you think she is.
These grapes are less expensive than those.
A donkey is less beautiful than a horse.
Would please tell me why "less ~ than comparison" is not regarded as good expression? Is there any grammatical background?

[konungursvia]

For starters, "less stupid" is a double negative, conceptually, as it means "less lacking in intelligence," which is not as clear as "smarter," a simple affirmation.

Also, it is less polite than the others, as it means you still consider him stupid, only less so. The others can be construed as mild compliments.