Does stationary bike excersice reduce stomach flab?

In short, yes. Although it's worth mentioning that you can only reduce your overall body fat percentage. Unlike targeted muscle gains, you can't target fat loss. That's right - you cannot target where your body loses fat. Your body stores it how it stores it, despite all the nutty pills and machines you see that claim to target stomach fat. Doing crunches will help by tightening up the muscles that exist beneath your stomach flab, but won't get rid of it. Fat has to be burned from your entire body through diet and exercise. and of course, you can't exercise away loose skin if you have had kids or lost a lot of weight.

Please remember that while we are responsible for keeping ourselves healthy, it is only the genetically lucky and those who work really hard who get to have it all. Skinny people may be called "hangers" but they have no shape. The curvier among us tend to also have curves where they don't want them. In my opinion, the best looking people are those who manage to maintain a moderate, healthy lifestyle. It's much better to look vibrant and vital because you're in good shape, than to look thin and sick because you didn't eat and then strapped yourself to the exercise bike for two hours.