If you omit the crumb topping, please use 1 cup (250g) of dairy-free milk in the batter. The topping shields the muffins from the oven heat, so they don't dry out quickly. However, baking the muffins without the topping can result in drier muffins.
To make a simple icing for your muffins, combine ¾ cup (75g) of powdered sugar with 3-4 teaspoons of dairy-free milk in a medium bowl. Add more sugar for a thicker icing or more liquid for a thinner icing and drizzle it over the cooled muffins.