[ETL]

Looks almost like dunes, but it can’t be because lack of atmosphere/moisture.... Volcanic or seismic related features I guess.

Like with many planets, moons, and large asteroids, covered with craters, the surface likely has a lot of loose material on it (sand, etc) from it being pulverized by impacts. A top surface layer of loose, unconsolidated material is known as regolith:

But, assuming for the time that it is sand, what caused the apparent "ripples" in it on Pluto?

[Boogieman]

Shockwaves from impacts maybe?

Even if there is no seismic activity native to the planet, a large impact would send waves out just like an earthquake...