Is it considered a mosque of detriment (Masjid al-Dirar) if it was built despite people's opposition, was close to an existing mosque, while there is another area in the same neighborhood that needs a mosque, and most of the people in the area do not pray in it after its opening?

It is not prescribed to build a second mosque near an existing one as long as the first one suffices for the worshippers. If the second mosque is intended to harm the first, then it is a masjid al-dirar (mosque of harm), and prayer in it is valid, though disliked. However, if harm is not intended and there is no need for it, then there is a difference of opinion regarding its permissibility.