Does wearing modest clothing, such as an Afghani qamis, in an environment where Western attire is prevalent, constitute a "libas shuhra" (garment of fame/ostentation)?

If modest, permissible clothing is available in your country that covers the 'awrah and does not resemble the clothing of disbelievers, then you must wear it. This is because going against the customary dress of the people of the land is a form of ostentation (shuhrah) and harm. What constitutes "clothing of ostentation" varies with time and place. Its defining characteristic is that a person wears clothing different from their usual attire with the intention of gaining fame, or clothing different from that of the people of their land without necessity. The default rule for clothing is permissibility, except for what the Shari'ah has forbidden. Trousers and shirts are not exclusive to disbelievers.