Tile backsplash mortar base coat?
I have laid a mortar skim coat many times to even out an uneven surface. I would use mortar not plaster as you have seen what happens to sheetrock when it gets damp plaster is the same. Make sure to cover and check your tile width just to make sure the evened up area matches up with your tile I usually feather 2-3 times the tile width so there are no noticeable height differences.