Zoro has enough other scars that those scars aren't even noticeable anymore. It's the huge gash from shoulder to hip that tends to draw the eye these days (and sometimes he thinks Mihawk actually did him a favor there).
Still, he's always a little nervous with his shirt off, especially with her around. The rest of the crew, they wouldn't even know what they were looking at, but Robin knows stuff.
That's not the only reason he distrusts her, but he wishes she would stop looking at him like she's trying to figure him out. Or like she already has.