It's irony... The hat is _**huge**_ and _**floppy**_ , and the speaker ironically attributes its eccentricity to the 'art' of making hats.
Cambridge defines art as
> _**the making of objects**_ , images, music, etc. that are beautiful or that express feelings
and
> a skill or special ability.
Another example of ironic use is
> Getting him to go out is quite an _**art**_ (= needs special skill).