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Irish UFC lightweight champion McGregor, 29, will make his boxing debut when he faces undefeated American Mayweather on 26 August in Las Vegas.
"There is tremendous build-up and hype but I don't really take it as a serious fight," Briton Lewis told Sportsweek.
He added that "nothing is ever obvious when it comes to gimmicks and hype" but "Mayweather should win it".
Mayweather, 40, will surpass fellow American Rocky Marciano's perfect record of 49 fights without defeat should he overcome McGregor.
But Lewis told BBC Radio 5 live: "I wouldn't count a fight like this as a 'fight'. He's basically boxing a guy that is a UFC fighter and not a boxer.
He added: "Everybody wants to know what's going to happen.
"Is McGregor going to throw a kick because, obviously when he resorts to boxing and realises he is no match for Mayweather, what will he do? How will he get around that?
"No other boxers from 49 fights have figured out how to beat Mayweather - now a UFC fighter that doesn't even have a boxing career is trying to beat him at boxing - I think it's a farce to a certain degree.
"I don't think it is a fight that I am going to jump on a plane and go to see unless they pay me to commentate on it."
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BBC Radio 5 live boxing analyst Steve Bunce:
It's going to be a relentless week. Too many people are treating it like a comedy carnival. On the night I think it will be a lot more serious than people think. I think the laughter will stop in Las Vegas, probably around Wednesday.
McGregor is a guy who was nowhere four years ago. Now he can earn hundreds of millions of dollars. He got there because he is absolutely driven. I've seen his fights, I think he likes hurting people. So there will be dark undertones next week. Anyone who thinks Mayweather doesn't have a nasty, dark side has been living in a cave for 30 years.
I believe McGregor has picked up a lot from Mayweather. If you are skint like he was five or six years ago and you are seeing a guy like Mayweather who has been making it rain throwing dollar bills in the air, of course he is copying that. He has wanted a bit of that. I can imagine McGregor before he goes to bed at night doing a few Mayweather moves in the mirror.
Floyd Mayweather's fight against Conor McGregor is a farce, says former world heavyweight champion Lennox Lewis.