I have lasered text with both thick and thin lines onto leather. Both can work well based on the contrast of the chosen color combinations. Where the contrast is weaker, such as a brown and black material or a black and gold combination, thin lines are harder to see.
However, on the light brown (tan), pink, or gray finishes, the black base displays thin lines well. Graphics such as logos with thin lines have the same results.
Keep reading about laser engraving leather.