There is a hidden meaning to this quote. Hachiman is implying that nice girls are lies, and that cruel mistresses are the truth. While girls who are nice do indeed exist of course, the charismatic ones who are considered "Nice girls" tend to exist truthfully only in fantasy.
That is indeed what he is implying. Because sometimes, people hide under a facade in order to be liked and accepted by those around them. Facades aren't entirely bad, because some people put up a facade to protect themselves; though there are those who put up a facade to fool others.