Should one leave the mosque if the Imam and the congregants engage in collective remembrance (dhikr), or is it permissible to distance oneself from them at the back of the mosque?

If the congregation consistently engages in communal remembrance (dhikr) after the obligatory prayer, this falls under the category of an additive innovation (bid‘ah idafiyah). They should be advised gently, and one should not participate with them in that innovation. It is preferable to sit in another part of the mosque, and it is not obligatory to leave it.