No, it's neuter locative. Coś (something) is neuter. O (about) with verbs like mówić o... (speak about...), rozmawiać o... (talk about...), myśleć o (think about...) requires the locative case.
The endings in this case would be: "nudnej" (feminine), "nudnym" (person-masculine, inanimate-masculine, neuter) and "nudnych" (plural both).
What about 'He speaks of something boring'? 'Speaking of this', 'Speaking of that' - after 'speaking' I would usually use 'of' rather than 'about'.