No, it's not, at least not in an urban environment with traffic and red lights (assuming you don't run them). Cycling is a non-load-bearing exercise. Overweight people, having excess weight, would stand to benefit much more from having to support their excess bodyweight during exercise than sitting the whole time. And being non-load bearing, cycling does nothing for bone density, and if done to excess, actively harms it: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3230645/