The phrase "disappointing season" means that a sports team or individual did not perform as well as expected or hoped for during a particular time period. Full definition
I think this is by far my most disappointing season in 30 odd years as a fan. (justarsenal.com)
It was a very disappointing season and a wasted opportunity. (sbnation.com)
His did not continue such a rich vein of form, however, and in a largely disappointing season he scored just 9 goals in 30 appearances. (westhamway.net)