It's ironic because he didn't have acting experience and they wanted him off the show in the beginning now he is one the biggest male actors.

He may have made millions from his music prior to the start of the series but he was almost bankrupt. During the first 3 seasons 70% of his income went to the IRS. He needed the series to pay off his debts.

Well Will Smith had already made millions as a well established rapper having made three albums and winning a Grammy and an MTV Music Award previous to TFPoBA. He also went on to win three more Grammys and a whole pile more other awards.
My point is he was well on the road to riches through music and possibly had a lucrative acting career ahead with or without the Fresh Prince character.
If he was an unknown young actor who caught a break with the show then the irony theory might work.
We have to show the world that not all of us are like him: Henning von Tresckow
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It's ironic because his character is from a lower class background sent to live in one of the wealthiest neighbors in the America. It wasn't until after the show ended in 1996 that Will started starring in major films.

So where's the Irony? He was the star and went on to act in major films.