There's no such thing as 'healthy' or 'unhealthy' food (with obvious exception of anything that's actually poisonous, of course!). There are only healthy and unhealthy diets.
There's nothing particular to worry about with eating corn flakes, except that you need to be aware of the fairly high salt content. (The Kellogg's website states that a portion of their cornflakes contains 9% of your guideline daily allowance of salt. Other brands may contain quite a bit more:
http://www.kelloggs.co.uk/products/cornflakes/Cereal/corn_flakes.aspx ). That doesn't mean that they're 'unhealthy'; it simply means that you need to watch the rest of the salt intake in your diet.
Chris