No, in mine, they do not fill the nest box. They scratch the stuff under the nest box and next to the nest box. Since they do not have good access, they cannot scratch it away so it stays built up. I would like to see a picture of what you have. It sounds like it might be a little different than what I'm talking about, but I suspect it is more that they don't have good access to scratch it away more than they purposely built a ramp.
I don't think chickens are dumb. They can actually learn pretty quickly. But I think they are more instinct driven that we are and their instincts are different than ours. Their reasoning ability is not always real good either. An example, a few days ago, one of mine got inside a fenced area that had a gate open. When it came time to go to the coop for nighty-night, the hen went to the corner of the fenced area nearest the coop and kept trying to get out through there. Although the gate was still open, she did not reason out, I came through the gate so I can go back the same way. Her instinct was Get to the coop! Get to the coop! I could not even herd her to the gate but had to catch her and physically put her out of the enclosure. I have no doubt she could be taught to use the gate, but her instincts strongly overcame her reasoning abilities.