When aircraft displaced the battlecruiser as the primary fleet scout

Could carriers and small cruisers already perform Lexington's scouting job better in the 1920s?

Not reliably at first; aircraft superiority emerged gradually, after which a Lexington could shift from scouting to shielding the carrier that took over reconnaissance.

Aircraft would eventually scout far better than Lexington, but Second World War reliability cannot be projected backward into the 1920s. The transition is difficult to date because evidence outside major-power exercises is limited for the 1920s and 1930s. Lexington could then move from primary reconnaissance asset to an enabler that protected the carrier conducting reconnaissance. Renown's service with Ark Royal illustrates the later role: even if unable to defeat every enemy fast battleship, a battlecruiser could deter pursuit or sacrifice time so the carrier escaped, a protection that light escorts could not provide to Glorious.