You can do it like in the example below:
DECLARE @x FLOAT = 0.199786581197469;
SELECT CAST(CAST(100*@x AS DECIMAL(19,2)) AS VARCHAR(32))+'%';Most experts would advise that you don't create a function to do this - rather, just do it in-line, for better performance.
Also, in general, you would want to do the formatting and adding percentage symbols etc. in the presentation layer (such as a client application or reporting services) if you have one, rather than doing it in the SQL query.