subsurf gives bad results when not clean topology. like any tool you have to understand what it does. it tries to subdivide every edge to the amount you have set it up.
subsurf works best with quad only topology, that said, you can use it with tris and ngons even, however you need to be mindful when doing so.
best examples is that you have single plane or box and put subsurf on that and see how it divides the mesh. you can also just make a plane with 3 vertex and subdivide that and see how blender will do it.
looking at the pic, to clean things up, you’d need to add geometry to control the subdivision.
