A professor commented that my research is too simple as compared to my colleagues. What does that mean about my future prospects?
Perhaps the best route is to split the difference. Note the concern of one professor, but take heart from the support of your advisor. There are a lot of possible explanations. 

Perhaps the professor making the comment is overlooking some aspects that are harder than they think. Some things that look easy from the outside are harder when you get into the details. And, perhaps, that professor is just so brilliant that the crux of your problem seems obvious to them. There are such people, of course. 

Perhaps your current research really is simpler than that of others in their circle. But you are still early in the profession. You will probably do better work as you advance. The dissertation shouldn't be viewed as your life's best work, only the first. 

There is no real reason to worry at this point, and certainly not to panic. But long term, consider the advice you were given. Perhaps a bit more experience will help you judge its merits or lack.