The expression, "If not now, then when?" is most familiar to English speakers as a maxim attributed to the Jewish sage Hillel. An early rendering of Hillel's statement appears in "The Condition and Belief of the Jews at the Time of the Coming of Christ," in The Biblical Repository and Classical Review (July 1839):
Only some particular sayings, characteristic of each [Hillel and Shammai] have come down to us [from the era of Herod]. Thus, Hillel inculcated, as the fundamental principle of Judaism, this maxim: Love thy neighbor as thyself. On the necessity of the early prosecution of knowledge, with his accustomed brevity, he said: Unless I for myself, who will? If I only for myself, what do I become? If not now, then when?
The sense of the phrase in Hillel's use seems to be something like an exhortation not to put off the task of self-improvement.
A similar phrase appears in a sermon by Lancelot Andrewes, "Of Repentance, and Fasting," preached before King James at Whitehall on February 26, 1723, in XCVI Sermons, second edition (1632):
And so would hee [St. John] have it, now : For, now (saith he) is the axe laid to the root : Now then, or not at all. Nay, not now : this is not a time ; we have appointed other businesse which we cannot put off. Well, one question more will make an end ; if not at this time, at what time? If not now, when? But then, this must be set downe, now before we stirre hence ; And so set downe, as if it be not now, it be as neere now, as may be, for feare ventura come not too soone, and take tree and all. This is sure ; the sooner, the better, because the more likely ; the later, the worse, because the less certaine.
Here the call to action appears to be to begin without delay to put one's spiritual life in order, because otherwise the end may come before one is ready for it.
In my view, whether understood as a religious message or as a purely secular one, "If not now, [then] when?" is an assertion about the urgency of prompt action. I do not see it as having intrinsically bad connotations. Rather, it expresses an orientation toward life that advocates action, change, and improvement. You could certainly choose worse sentiments to commit indelibly to your skin.