Mendix 5.21 has the Text widget, which allows you to format one or more attributes into a text value.
It sounds like you actually do not want to use a float here, but you should use a Decimal instead. A Float cannot be used to accurately represent decimal values, while a Decimal can (which is the reason why Mendix introduced Decimal and got rid of Currency, which was in fact a Float).
You could try the microflow label widget. Does not store an attribute, but needs a microflow to format your float as a string
Depending on your use, you can probably accomplish this with CSS as well - using the :after selector to add a % sign. Here's a sample that does the same with the :before selector for currency.