Is it permissible to give zakat to a maternal uncle who is indebted and unable to pay off his debt, and is it permissible not to inform him that this money is from zakat?

If the maternal uncle is poor and eligible for zakat, it is permissible to give him from the zakat funds. It is not a condition to inform him that it is zakat, unless his condition is not well known, in which case he should be informed so that he may refrain from taking it if he is not eligible.