No one would want to be famous who hadn’t also, somewhere in the past, been made to feel extremely insignificant.
Is the sentence a double negative?
Is “who” a relative pronoun which modifies “one”?
Is the subjunctive mood be used in the above sentence?
Yes, “who” relates back to “no one.” To illustrate, I’ll bring the subject and relative clause together in a slightly expanded way:
No one (…) who had not also (…) been made to feel extremely insignificant.
In this case, the statement should be read as a double negative where the negatives cancel one another out. (Merriam-Webster discusses the tendency to read double negatives “mathematically” in a Usage Note; it may also be litotes, described in the same post). Semantically, it should be read like this (excuse the awkwardness of the rephrasing):
?One would want to be famous who had also, somewhere in the past, been made to feel extremely insignificant.
As to your final question, I see nothing that would distinguish the subjunctive mood from the indicative. Would want seems to be in the indicative, expressing an inclination. Hadn’t also … been made to feel is in past perfect; the subjunctive and the indicative are not distinguished in this tense. Given the absence of formal differences and the fact that the situation described is not clearly unreal in the way these sources suggests, I would not assign subjunctive mood to it. However, this point is a bit fuzzy and might warrant a question on its own.