While a lot of the play's comedic flair comes from Lady Brit's lines, her husband, Andrew Undershaft, is actually at the heart of the moments that signal the play's overall tone. Like the play itself, Undershaft is good natured and humorous, but he doesn't suffer fools gladly. For example, when he, Lady Brit, and Stephen are discussing Stephen's career prospects, he responds to Stephen's overconfident idealism by observing acidly to his wife that Stephen "knows nothing and thinks he knows everything. That points clearly to a political career" (3.127).
The play often gives us lines like that—they're enjoyable because they're clever and humorous, but they also poke at some apparent hypocrisy or contradiction in a character's views. The cleverness and humor helps soften things, but there's still some biting commentary on society/human nature going on here.