Since the play was inspired by an English language primer, Ionesco first thought of calling it English Made Easy. We're glad he didn't. That sounds kind of boring. Next he thought of calling it The English Hour. Whoa, even more boring. According to legend (we're not sure if this is true), the actual title came into being during a rehearsal when the actor playing the Fire Chief flubbed a line. He was supposed to say "une institutrice blonde" (a blond schoolteacher), but instead said, "une cantatrice chauve" (a bald soprano) (source). We're guessing Ionesco thought this mess-up was awesome. Not only did he add a line as a result, he even named the play after it. The mention of the bald soprano goes like this in the completed play:
Fire Chief: [moving towards the door, then stopping] "Speaking of that--the bald soprano?" [General silence, embarrassment]
Mrs. Smith: "She always wears her hair in the same style."
Fire Chief: "Ah! Then goodbye, ladies and gentlemen." (472-474)
What in the world is going on here? Why is everybody embarrassed that the Fire Chief mentions the Bald Soprano? And why does Mrs. Smith's line seem to satisfy the Fire Chief in some way? There's really no telling. In his essay, "The Bald Soprano: A Tragedy of Language," Ionesco says, "one of the reasons why The Bald Soprano received this title is that no prima donna [soprano], with or without hair, appears in the play. This detail should suffice" (source). Thanks Ionesco, that's not very helpful. Sometimes we think he's deliberately messing with our heads.
Basically, we advise you to not stress out too much trying to figure out the title of The Bald Soprano is supposed to mean. The play is purposefully full of all kinds of nonsensical things. The pertinent question is probably not, "Why is the play called The Bald Soprano?" but instead, "Why shouldn't it be?"