Cornflakes are generally a simple cereal absent very high levels of sugar and salt. That said, not likely particularly nourishing but a piece or two once in a while shouldn't be harmful. Please check the label to ensure no other hazardous ingredients.
Only my personal opinion - I wouldn’t. I tend to stay away from things with added sugar and salt. The added vitamins and minerals would also give me pause, I’m not sure that iron and zinc would be good additives for a bird. Although I’m not sure that there is enough in there to cause concern, I tend to stick to more natural treats. Pine nuts, seeds, bits of fruit, greens, etc is what I generally use.
Many cereals (99%) are fortified with added vitamins and minerals which are added in quantities appropriate for humans. By feeding these to birds, you run the risk of overdosing them vitamins. It will obviously depend on the quantity given and the length of time for which the cereal is fed, but I do not give my bird anything with added vitamins and minerals (excluding those foods that are intended for birds, and even then, I am cautious).
I give my Cockatiels a pinch of Grape Nuts flakes.
Very little added sugar. I am sure it has added vitamins and what not BUT the amount is based on a HUMAN serving like a cup or bowl full, not a small pinch for a bird.