Air Traffic Controller Johnny isn't an essential character in Airplane! He isn't crucial to the plot. He doesn't help advance the story in any way.
But boy can he land a joke.
In his limited screen time, he manages to crack as many memorable jokes and one-liners as anyone in the movie. He's colorful, animated, and adds a bit of light-hearted lunacy to a control room dominated by stiffs and cynics.
Johnny's especially notable because in his flamboyance and decidedly non-deadpan approach, he has the distinction of being the one significant character in the entire movie who doesn't play it straight—pun intended. In this sense, to some, Johnny represents one of the movie's more problematic parts: an offensive stereotype of a gay man that hasn't aged well, time-stamping Airplane! firmly in the distant past. (Source)
It's also possible, however, that Johnny is actually progressive for his time, a character that is unapologetically gay in an era where that was still taboo. He isn't harassed by the caricature manly-men characters that dominate Airplane!; instead, he turns the joke around on them, providing the perfect foil to their sternness and self-importance.
We come to understand Johnny a little better when we take a look at the actor who played him. ZAZ developed the character explicitly for actor Stephen Stucker, who wrote his own lines for the part. Stucker has a notable legacy as one of the first openly gay actors in Hollywood at a time when there was serious paranoia around the AIDS epidemic. A strong-willed and eloquent advocate for LGBTQ rights, Stucker ultimately died from AIDS-related complications in 1986, but not before fearlessly challenging stigma and raising awareness about the disease.
So is the character of Johnny offensive? You be the judge.