It's not Plural. Gemüter is Plural, but wenige qualifies kritisch. You can be less or more critical. In this case, less. Hence it's weniger.
Would be the same, if the object they are referring to was Singular, e. g. "Für ein weniger kritisches Gemüt..."
Plural would be wenige. But then it would qualify Gemüter. "Für wenige kritische Gemüter..." would mean there are few and critical minds, not less critical minds. It would probably be understood to mean that it was reassuring to few critical minds, i. e. not reassuring to many or even most critical minds.
Wenigere is comparative and doesn't fit in here.