Definitely looks like sunburn You can't move plants that have always been in shade straight into full sun. They need to be adjusted slowly. Does that container it's in have good drainage?

How much direct sun should a baby mango like this get?

I punched a bunch of holes at the bottom of the jug. Drainage is good as far as I can tell.

There's a few variables that can affect it. Under high humidity their tolerance is greater. Also with good air movement. If it's in the ground, temperature at the soil surface is very much higher than say at human head height. A very small plant cops most of that heat which is very high when the sun is out.

Keep it in filtered sun (like under a tree) for a while and then slowly increase the sun exposure, particularly in the mornings. New leaves should be able to cope with the increased sun. If you look at new leaves, they usually hang straight down to start off with, gives them less direct sun during middle of day. As the leaves mature they lift up to get the full sun most of the day.