Do family support payments through a payment app count as taxable income?

Usually the app is not the deciding fact. The transfer's purpose is.

If a taxpayer sends money to a family member out of detached generosity or support, and the recipient did not provide services, sell property, or promise repayment, the transfer is generally analyzed as a gift rather than taxable income to the recipient.

Fresh example:

• Elena Marsh sends her uncle 900 each month after his hours are cut.
• He does not work for Elena.
• There is no loan agreement.
• Elena is not reimbursing him for a shared bill.

That looks like family support, not wage income. The payment app creates a record, but it does not convert the transfer into income.