Usually directly feeding animals is looked down upon because decreasing their fear of humans tends to be a negative for their survival, and it might impact their ability to forage for their own food. With birds specifically, though, putting up feeders is more mixed. Migration takes an enormous amount of energy and and human sprawl has removed a lot of natural food sources. And especially in wintertime, food can be quite scarce for birds. But at the same time bird feeders can actually be big spreaders of disease and I know that there was guidance that people should take down feeders at the height of bird flu.

Now when it comes to mallards, they're honestly a species that is incredibly urbanized already, so I don't think directly feeding them is doing a great deal of harm.

Those are basically already domnesticated if they live in a city.

depends on if you want them to like you more than you care about ecology

There are signs by me that say not to feed migratory species in winter I believe. I presume the food makes them stick around when they would otherwise migrate.