It really depends on what you want to do. I think it's fair to use placeholders as long as they are placeholders for you to fill before release.
What does that mean? Use them if you need to build a map if it's easier for you, but try not to upload a demo with them. Even if your plot is good and your mapping is good, it will distract players too much and irk them the wrong way, which could potentially damage the hopes they had for your game, as it's seen as a lazy move.
Even if your own work is bad it adds a unique character to it that's still better than the same copy pasted recolor shit.