Absolutely, go ahead and make them on your own, especially if you're trying to go to a popular place!
We had a travel agent last time who was great at everything... except the ADRs. A week after the 180-day window opened, she casually e-mailed me for a list of the ADRs we wanted so she could make them. I told her I had already done so -- at 6:00am a week before. Her relaxed attitude was probably appropriate considering that we were traveling during a low crowd season without a lot of competition for ADRs, but I was still surprised that she was planning on doing it so late. (We had never discussed who was making the ADRs - I had assumed from the beginning that although she was funneling information to us about the restaurants, it was my job to make the ADRs.)
That experience is what pushed me toward making all of our arrangements myself from now on. I think going through a travel agent is great if you're new at Disney vacation planning, don't want the headache of making arrangements yourself, enjoy having someone to collaborate with on your plans, or don't want to have to keep up with every discount and refurbishment that is announced in the months leading to your trip. Since I actually ENJOY that level of involvement and obsessiveness, we really don't get much of an added benefit from using a travel agent, although I'm glad we did it at least once, and our agent was a delight to work with.