Freestyler can mean many things. As in singing/rapping freestyler means you make it up as you go along. Best example is Eminem in 8 Mile. All the "battles" were freestyling.
I take his Free Styler T-shirt to mean that he is Free and unique and a styler as in he is a trend setter, not one who follows the crowd.
I liked the other t-shirt he had on the other night with three seperate pictures on it. With a man and a woman (the kind you see on toilet doors for male and female and it says good. Next to it, it has a bloke with woman either side and it says underneath, better. The last pic is a stickman playing a games console and underneath it says even better
Nothing on Marios T-shirts though
Which incidently are all from a company called tinypenis.co.uk