Why do babies have hard time breathing when congested? specially when sleeping & feeding?

The babies airway is blocked. See a pediatrician ASAP.

A mucus layer is a standard thickness at any age. The nasal airway of an infant is tiny compared to older kids or adults. Think of air passing through a 1 inch pipe vs a straw, the same mucus layer can block the straw while it does little to block the pipe. A vigorous saline cleanout of the infants nasal airway is often effective in reducing the blockage & helping them breathe.