Do I have to like everything about Japan to naturalize?
No. The naturalization process for Japan doesn't have a formal written cultural or ethnic "assimilation" or "like" requirement. It does sometimes have a basic-level Japanese "language" requirement, but this requirement is born from the rule that an applicant be able to demonstrate that they can earn a decent living in Japan — "decent" being not wealthy or rich, but not likely to need government welfare or likely to end up in poverty.
The modern Constitution of Japan enshrines a person's freedom of expression and the freedom to lawfully assemble and protest. These rights were inspired by the same rights in the U.S. Constitution. The laws created and the supreme court judgements related to this Constitutional right (Articles 19 and 21) can be argued by legal scholars as being slightly more limited than the U.S. (for instance, with respect to libel and slander), but broader and more open than the freedom of expression allowed by other democracies, like Germany.
The only requirement that the naturalization process has regarding "not liking" Japan is that if you do it, you do it in a lawful (i.e. not violent or by force) manner. This includes not associating or linking oneself formerly to groups that have acted unlawfully (again, unlawful violence or force) against the modern government or Constitution. So if you attend a demonstration against something in Japan, make sure it doesn't devolve into a riot, and the organizing group has properly applied for permits, etc.
There is one special case (that is explicitly mentioned in the application requirements) where you may be allowed to participate in a group that has attempted to overthrow the government or Constitution against the laws of Japan: those who participated in World War 2 or other wars to fight, directly or indirectly or subversively, by force or without force, against the former government of Imperial Japan (in other words, before 1947). There are many examples of this happening. The most recent: Donald Keene, who honed his Japanese language abilities helping the U.S. and its allies defeat the former Empire of Japan in the Pacific War.
There is also an relatively recent case of democratic protest (in the form of public press and a lawsuit) in the English language naturalization sphere: 有道出人, former American, who successfully naturalized despite his case worker's awareness, due to national press, of his lawsuit and campaign against a racist exclusionary hot spring resort in Sapporo:
[case worker]: "I've been seeing you a lot in the papers these days about the Otaru Onsens thingie.So in conclusion, the Constitution of Japan (and thus, the nationality/naturalization laws under it) not only give you the right to be happy and content, you also have the right to be unhappy and discontent; you can both express those grievances and fight injustices in a legal manner. Even before you become legally Japanese.
…
[Arudou]: "Will that affect my chances of becoming a Japanese citizen?"
[case worker]: "Absolutely not," …. "You are doing this privately, not as a member of a known subversive group. You are not trying to undermine our country or our Constitution. This will in no way affect the decision making process."
I'm not addressing whether or not it is a good idea, from a personal life actualization point-of-view, to change to Japanese citizenship if you have issues with living in Japan. It's possible that your case worker, if he or she detects that your attempt to naturalize is some form of masochism — for example, by reading your 動機書 (motivation essay) — may go off-the-record and ask you to think about the consequences for your own sake.
Similarly, you can legally get married to a person even though you dislike or even hate the person. You have the right to do that. Whether or not that's a good idea (for your own sake) is an entirely different question.
Just like marriage, undoing a naturalization (renouncing your citizenship) is legally guaranteed. In Japan, it's even a constitutional right (article 22). However, just like there's no guarantee that another person will marry you again after a divorce or your original partner you divorced will take you back, there is no 100% guarantee that another country will grant you citizenship or your original country will undo your renunciation or grant you citizenship again via naturalization or another process. It depends country by country.
I'm sure that all naturalized citizens have things they don't like about their adopted country, from everyday annoyances to perhaps politicians or laws. However, because naturalization can potentially be final — and even if it isn't, it is a very hard process to do and undo — you should self-reflect very carefully as to whether or not the day-to-day things that bother you don't cross an internal line and will keep you from exercising your right to the 幸福追求 ("pursuit of happiness") as per Article 13.
