Yes, you're doing tons of good by exercising after dinner! When you stop and think about it, you'll realize that dinner isn't even digested thirty minutes or an hour after dinner -- it's still in your stomach. So dinner can't be what your burning off since it hasn't even been broken down yet.
Exercise, especially cardio exercise, burns calories - not individual meals - and helps create a calorie deficit that forces your body to burn stored fat. I know that some people say to do it first thing in the morning on an empty stomach -- others say that's a myth. In my opinion, it really doesn't matter a whole lot
when you do it -- all that matter is that you're doing it!
The only downside to exercising right after eating is if it makes you queasy. That's why most people wait for an hour and a half to two hours after eating to exercise -- but you gotta do what you gotta do! And if exercising after dinner is what works for you, then go for it!