The BOPPickaxe implementation sets the HarvestLevel to 4: https://github.com/Glitchfiend/BiomesOPlenty/blob/BOP-1.7.10-2.1.x/src/main/java/biomesoplenty/common/items/ItemBOPPickaxe.java#L19

This means that the toolMaterials harvestlevel is ignored when querying the harvestlevel. The mud pick therefore has harvest level 4 instead of 0.
Simply set it to toolMaterial.getHarvestLevel() instead of 4.