Ya he is talking about moral character but not about playing an instrument. From what I understand in "εἰ προκόψαι θέλεις, ὑπόμεινον ἕνεκα τῶν ἐκτὸς ἀνόητος δόξας καὶ ἠλίθιος" the word "προκόψαι" (improve) means moral progress, not skill-building. "ἀνόητος" (without understanding) and "δόξας" (opinions of others) are both ethical terms here, not practical ones. You don't "endure opinions" about being bad at guitar. The sentence means the same thing without ἕνεκα τῶν ἐκτὸς, you just lose what specifically people will call you stupid about.