Adrian, 

the direct object of the ( very ‘familiar’ ) verb ‘ se mater ‘ in this sentence is ‘ toute la saison ‘, and follows the verb/past participle, so no agreement of the past participle. 

With regard to ‘ un ‘ versus ‘ une ‘ : 

 this is abbreviated speech - shortened version of ‘ ( la saison) numéro un ‘ which is standard, although wordreference notes it can used in the feminine as well. 

 

 

https://www.wordreference.com/enfr/number%20one

   https://www.wordreference.com/fren/numéro

 https://www.larousse.fr/dictionnaires/francais/numéro/55259