In rome remaster do cities start making better agents if they build alot of one kind?

I am wondering because I think in Total War: Medieval II it worked that way.

Its possible, as they did import some of the agent stuff from Medieval 2. I think I did see a gradual increase in agent quality as the game went on, but that might also have been because my buildings were being upgraded at the same time.

Does that actually have a significant effect or is it just marginal improvement?

In the original STW and MTW, anything below 3 stars as an agent was a random chance.