If you are "ang moh pai" one, then no it is not. I am "ang moh pai" one... And my baby drinks mainly formula and eats 2 cereal feeds a day plus 1 porridge feed. He drinks only 20ml of water in a day and he is doing ok. So far my baby has no constipation problem. Last time when he was on full breast milk, he poo 4 times a day. Now he poo 3 times a day.
That time I also did ask the pediatrician coz my son dont't drink water and eat a lot of "heaty" things like cereal and forumla milk. But pediatrician says it's fine. Formula is like liquid intake but he says it is still best to try to let him get accustomed to water just so that next time he doesn't need to depend on "tasty" drinks to drink liquids. Same case for cereal.
I think cause of constipation would be more of lack of fibre in baby's diet rather than the cereal. I always try to mix some puree vege into his cereal or some fruit so that there is fibre in his diet to help him poo.