Almost, but you will only get the 'stdout'. To get the error output use:
smbmount //Yod4z/Pc /mnt/ext -o username=administrator,password=pass 2> mnt/sd/log.txt
or to get both standard output and error output use:
smbmount //Yod4z/Pc /mnt/ext -o username=administrator,password=pass &>...