By clutter do you mean tree type stuff or grass.
For clutter/grass you can modify both the clutter it's self and what type and where on the terrain. Also the selection of type to random what type.
For trees you can not change the basic tree set / city building stuff but you can modify there placement on terrain tiles and square objects.
HiTech
By clutter, I always mean trees, building and grass as opposed to a bare ground type tile. Anything that obstructs your view or movement.
I get that we can't change the tree set, , but do you also mean we can't add user's custom objects to tiles? Keep in mind I won't know how my idea will look until I try it... I was thinking of very short LOD screening saplings in the 100 to 200 feet visible range for GV battles with compensating aerial views, pictures of denser forest scenes on the diffuse ground tile. I have in mind no-collide 2D objects to infill certain tiles, forest and jungle. At ground level, twenty or thirty feet high saplings etc. don't need to sway in the breeze. If it doesn't work, it wouldn't be the first time I've tried something that failed.