Is it appropriate to mourn or conduct Majlis, recite Noha, and perform Matam during weddings and Eid, considering that some Shia believe that Eid coincides with the 10th day of Imam Ali's martyrdom?
Bismihi ta'ala
Unfortunately, this is a clear misguided view and practice. In the dua we recite on Eid al-Fitr day, it says it is a day in which Allah ta'ala made it as Eid for Muslims. It is definitely not appropriate to change a happy celebration occasion to mourning and ma'tam. There is a time for each of them.
As the hadith says, "Our Shi'ah..... they grieve when we grieve, and are happy when we are happy."
Much more can be said about this, and the most important thing is one follows not what a small fringe group does, but what majority of Shi'ah do, especially our mainstream esteemed 'Ulama.
And Allah knows best