As compared to say:
REPLICATE(@padchar, @len - LEN(@str)) + @str
This is simply an inefficient use of SQL, no matter how you do it.
perhaps something like
right('XXXXXXXXXXXX'+ rtrim(@str), @n)
where X is your padding character and @n is the number of characters in the resulting string (assuming you need the padding because you are dealing with a fixed length).
But as I said you should really avoid doing this in your database.