As mentioned before, it seems bloody to us and a Allied success only with hindsight. Without hindsight it simply seems that the British were kicked off the continent in a 3-week campaign. The whole campaign happened so that the British won't set up airbases to bomb Ploesti. Now that the whole mainland fell relatively quickly and with ight casualties, Crete is very tempting. Especially since they don't need the 12th Army, just 3 divisions.
Whether the Battle of Crete takes place, I think is mostly a Luftwaffe issue. Can they establish themselves quickly enough to use VIII. Fliegerkorps before it is sent north for Barbarossa? Will they go ahead with just the 250 aircraft of X. Fliegerkorps and whatever the Regia Aeronautica can cough up?
If anything, the Corinth Canal can show that they simply used inadequate forces and there was no airfield nearby to capture and use to transport reinforcements. They can change their OTL plans for Crete: instead of scatter a single regiment to each airfield, they can double down: Use the remaining 2 regiments of the 7th Paratrooper to seize a single airfield (Maleme or Heraklion) and then transport there the 22nd Airlanding Division with the 5th Mountain behind. I believe they can seize that single airfield and transport the 22nd.
Not that it will matter: Crete cannot fall in TTL. The Germans will just lose a whole corps equivalent. The Commonwealth are in better shape with at least some heavy equipment. There are tanks with guns (and not just machine guns) that are in working order. There are 20,000 angry Cretan veterans.