First thing you can probably do to help this a lot is to work on the outline and form. For example since you're working with a small size it's probably best not to use lines to portray volume(such as the hem of the hat) and instead use lighting. A sharp jump from light to dark will imply a corner or edge.

Second, don't use pure greys unless you know what you're doing. A grey with a very light touch of hue looks much more pleasing to the eye. Typically when you're working with darks a hint of purple suits this well. In this instance blue could probably work, too.



After you work on those two things you can start shading. Try to avoid typical gradients. Get a little creative with it. For example typically shiny stuff(like a nice, shiny expensive tophat) could use "streaks" of highlights to make things a little more interesting. Also don't be afraid to play with these things a bit. For example instead of making the highlight go straight down you can make it taper off and disappear. This implies a conic shape to the hat and works with its shape to make it look more interesting than a very stiff hat, even if the lines still imply its edges are straight.

Don't be afraid to break away from the norm or logic to get creative, especially since it's for Graal that implements a cartoony style. You can often break off and do things that wouldn't make sense logically and it still looks nice and fits.