Do you really not understand how telling a woman that she looks like a bag lady is offensive, even setting aside the fact that a.) this was on a television program and b.) the woman was not in a position to say anything in response?
"Only one of the great cinematic costume designers would come to the awards dressed like a bag lady.
So lets break that down.
The subject is a "cinematic costume designer" According to Sir Fry, she is a great one at that.
Then we have the other part of the statement: "She came to the awards dressed as a bag lady." This part of the statement is a bit odd/stands out when compared to a group of rather posh individuals.
When you put both parts of this together you have a really positive reference by Mr. Fry then combined with an odd/"less flattering" situation. It's ironic.
What make this even funnier and more congratulating is: She's a costume designer. Maybe this was intended. At the end of the day Stephen Fry was pictured with the individual and there have been claims that there is no bad blood between the two.
To call it an offensive joke seems like you're perverting what was actually said and would require striping out the context.
Mr. Fry did not say: "Shes a bag lady, too poor to receive this award. Just take a look at her clothes"
That could be considered a joke at the expectation of being posh, at a highly honor awards ceremony, and would be considered offensive to the subject and maybe the audience.