Yes. The batter needs to be thick. The problem with people saying to fill above the halfway mark or 3/4 mark is that muffins can then end up a real mess. Undercooked or muffin batter that touches and so the muffins bake connected.I think it depends on how heavy the batter is too
Now THAT I like!My favorite part of a muffin is when streusel is on top ... Here's instructions I found on Allrecipes.com:
Combine 2 tablespoons flour, 5 tablespoons sugar ,and 1/2 teaspoon cinnamon in a small bowl. Cut in 2 tablespoons butter with fork or pastry blender until mixture resembles course crumbs. Sprinkle over batter in muffin cups.