All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
These people cannot be judged as making fun of Islam because they merely make fun of wearing the long qamees (thobe). It seems that the reason for which they do not accept it is that it is different from the clothes that are commonly worn among them, not because it is recommended to wear in the Islamic Shariah. So take it easy upon yourself. If you refrained from wearing the qamees and wore what the people in your community wear of lawful clothes in order to win their hearts and invite them to more important matters than clothes, then that would be good. You should know that the Muslim is not obliged to wear the qamees, whether the long or the short one; rather, it is only recommended. It was reported in the Sunnah that it is recommended to wear the qamees. Umm Salamah, may Allah be pleased with her, said, "The garment that the Messenger of Allah, sallallahu ‘alayhi wa sallam, loved most was the qamees." [Abu Daawood and At-Tirmithi]
Ash-Shawkaani wrote, "It is deduced from the hadeeth that it is recommended to wear the qamees." [Nayl Al-Awtaar]
Therefore, the scholars stated that wearing the qamees is recommended. The author of Kash-shaaf Al-Qinaa‘ said, "It is a Sunnah to wear the qamees, based on the statement of Umm Salamah: 'The garment that the Messenger of Allah, sallallahu ‘alayhi wa sallam, loved most was the qamees.'..."
Allah knows best.