The 3D Textured Mesh does not cover the full area of the model.
Pix4Dmapper does not generate the 3D Textured Mesh for points that are not very dense. The 3D Textured Mesh will not appear very nice for areas with sparse points.
A possible solution is to increase the density of the point cloud:
1. Set the density of the point cloud to High. For more information: Menu Process > Processing Options... > 2. Point Cloud and Mesh > Point Cloud.
2. Run again the 2nd step Point Cloud Densification: How many Points are generated during step 2. Point Cloud and Mesh?.
3. Generate the 3D Textured Mesh. For more information: Menu Process > Generate 3D Textured Mesh.
An alternative solution is to increase the amount of triangles to be generated, by increasing the value of the Sample Density Divider option as it is described here: Menu Process > Processing Options... > 2. Point Cloud and Mesh > Advanced.