As far as vocalists go, Ella isn’t “straight jazz.” When we talk of vocalists of this ilk, (Holiday, Vaughn, Washington, Sinatra, etc.) I don’t consider any of them “straight jazz.” I think of Abbey Lincoln who, by the 1960s, was singing real-deal jazz music with the great Max Roach (her husband). Brilliant.
Those previously mentioned vocalists are singing the songs of Jerome Kern, Cole Porter, the Gershwins, Harold Arlen, Rodgers & Hart, Irving Berlin, Rodgers & Hammerstein, etc.
These are some of the greatest pop songwriters in history, but not what I would call jazz.