"What an April Fool am I." If Cole Porter had written such a song, as well he might, surely Bobby Short would have sung it after his appearance at the Kennedy Center Concert Hall on Saturday evening. He should not have been there.

Badly staged, badly lit, and badly miked, he just managed to save Porter and Gershwin; they in turn just managed to save him; and a tolerant audience was willing to save them all. They improved the sound a little for the second half of his program - technicians scurried about the stage during the intermission - but this only emphasizes how little thought had been given to the evening, to the difficulty of presenting even as swell a bar entertainer as Short in such surroundings.

The huge, high, empty stage - whcih seemed to have been prepared for the execution of summary justice by a people's tribunal - dwarfed the small group used to the Cafe Carlyle. The drummer and the bass were hidden anyhow in deep shade. Short was himself dwarfed by the stage. He was like a doll on it. When he tried to use it, capering a little as he uncomfortably uttered a few program notes about the songs, it remained empty.

Of course the evening was just rescued. Bobby Short can do things with as slight a piece of skittishness as "I've a Shooting Box in Scotland" which no one else can. People applaud when they recognize the opening bars of one of the old favorites, but in a moment he can make one feel that one has never heard "Why Shouldn't I?," or "Sweet Embraceable You," or "'Smarvellous, 'Swonderful." But to have to sit up in an auditorium chair to catch the unexpected changes of tempo and phrasing with which he makes the songs his own is not 'smarvellous and is not 'swonderful. In short, it's not entertainment, because it should not be there, and the Washington Performing Arts Society should know it.