Mostly I see the usage of word Alibi in a negative context especially in the legal parlance. Following is the definition from Oxford Dictionaries:
1 a claim or piece of evidence that one was elsewhere when an act, typically a criminal one, is alleged to have taken place.
'she has an alibi for the whole of yesterday evening'
1.1 informal An excuse or pretext.
'a catch-all alibi for failure and inadequacy'
informal Provide an alibi for.
'her friend agreed to alibi her'
Idea of usage is, where internal quality of a human being / noun is depicted via another masquerading quality:
"Ambitious" is a convenient alibi for the "Ruthless"
Note: Two answers were based on a previous version of this phrase that read "Playful is an alibi for the Smart."