๐ I hope some rover drivers get to see this question - it's a very good one, just funny if you've seen the whole mission.
In the past - before Curiosity landed - NASA definitely chose rover landing sites based in part on their (presumed) smoothness and traversability (e.g. Opportunity). This was also true for the first Chinese lander.
In the case of Perseverance, the "rockiness" in this region actually varies quite a bit over fairly short distances. The terrain we've been exploring since late 2024 was chosen for two reasons: ease of traversal (when we were climbing out of the Jezero Crater) and science (our current location, Witch Hazel Hill). When we were down on the old river delta last year, though, the rover drivers had a very difficult time with terrain like this and this.
Witch Hazel Hill is smooth in part because the bedrock here is soft and easily eroded. Quite a bit of it has significant clay content, like you'd find in Earth soils, due to heavy interaction with water in the geologic past. Down on the crater floor where we landed, where the terrain is made of volcanic rocks, there are scenes like this. In the end, the rover drivers are pretty protective of their vehicle, so we tend to prefer smooth stretches for driving.
