Guitars look better if everything on them looks like it belongs on the same guitar. There is a tendency among some luthiers to load their instruments down with as much “gingerbread” as possible. While this is not exactly a crime, problems arise when (1) some of the decorations work against others, and certainly when (2) there’s much more there than needs to be. Think visual pleonasm.
Any individual inlay can be beautiful, but one can distract from another one, or they can clash. It is better if there is an inlay or highlight that “goes with” a second one and carries an echo of it. It doesn’t really have to be a separate element either; it can be an elaboration or enhancement of something that is already there. There is no limit to the possibilities, and in my experience the best results are gotten if one exercises a bit of restraint. I have never been able to make a guitar look good by yielding to the impulse to load it down with too many, and/or incompatible, visual elements.
Here are some examples of visual harmony from my own work.