I think lackey's point is that your wall appears to be a pile of rocks that are all kind of glued together or stuffed into an invisible cube, rather than looking like a boulder or an outcropping you might see in real life somewhere. This is not, obviously, to say that it should be realistic, only that it should be more believable. Most of your tiles right now have a distinct flavor of referencing other platformer games more than they are referencing art or real life, and I think because of that they are very scattered, abstract, and flat. You'll notice on both Lackey's mockups and Indigo's banner that the stone/rock/dirt tiles seem to form one cohesive surface, rather than a shambles of stones piled up like an old midieval hut of some sort. This is a more believable surface than a pile of stones that are sort of unbelievably vertical in their orientation.
At least for me, something that has helped my level art a LOT is to draw basically to-scale mockups in my little sketchbook, usually away from the computer entirely. I'll go through and doodle sections of the level, and any part that I'm having trouble executing int he sketchbook i KNOW i will have a lot of trouble executing in pixel art. For things like rocks, dirt and grass especially this has made a huge difference for me. Sketching is generally much faster than doing a fully realized pixeled test, and it can go a long ways toward helping you explore what works and what doesn't. It's an opportunity to think about and explore the materials and surfaces that you want to render without getting hung up on the particulars of palettes and AA and all of these trappings of good pixel art.
I feel like if you were just making a pencil drawing of some grass with a rocky outcropping you would not draw it like a pile of pebbles!