The 'rule' is that 1- and 2-syllable adjectives/adverbs preferably get the '-er' and '-est' endings, unless they are awkward or confusing to read or pronounce. I think that 'idle' falls under the definition of 'awkward, confusing', and I would use 'more idle', 'most idle' if I were doing it.
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GuestIs
GuestSuperlative/Comparetive
Is "idler" or "idlest" is correct?
or
Most idle