This will do what you want: Applies modifiers for all selected objects:

    import bpy
    import bmesh

    for ob in [ob for ob in bpy.context.selected_objects if ob.type == 'MESH']:
        b_me = bmesh.new()
        b_me.from_object(ob, bpy.context.scene)
        b_me.to_mesh(ob.data)
        b_me.free()
        
        for mod in ob.modifiers:
            ob.modifiers.remove(mod)

To add modifier (wireframe) for all selected objects:

    import bpy

    for ob in [ob for ob in bpy.context.selected_objects if ob.type == 'MESH']:
        mod = ob.modifiers.new(name='Wire', type='WIREFRAME')
        mod.thickness = 0.02 # SET THIS