The adjective flippant denotes a lack of seriousness or proper respect, characterized by an offhanded or casual demeanor. It implies an attitude where remarks are made lightly, without significant consideration for their gravity. In the given context, the comment about her clothing choice was probably not meant seriously, indicating a relaxed or indifferent disposition.
Evaluation of options:
Therefore, casual is the word that most closely approximates the meaning of flippant.