A common narrative I've seen over the conference is that it snubbed the Japanese because it gave the Japanese "only" 60% parity in capital ships to UK and US, a narrative also repeated in r/askhistorians here, I've seen it more commonly in the writings of lead up to WW2 of how the war hawks in IJN were outraged at the supposed slight at Washington. But I just don't see how that holds up.
First up, there weren't 3 participants at Washington, there were 5 and the other two, Italy and France got less tonnage than Japan despite the fact that France had a much bigger empire to run (although not a lot of money for new ships after WW1), was a much more established naval power and was a European empire, so I don't think the "white man betrayed Japan" narrative seems to very accurate since the French and Italians are after all "white".
And then, US and UK were the two biggest naval players in the world, Japan was barely able to keep up its naval construction programs despite taking up enormous share of budgets and couldn't really follow the 8-8 program properly, while Woodrow Wilson had just stamped the biggest naval expansion in American history (till that point) with a light pen stroke and Royal Navy while reeling from WW1 was still the largest navy and its projected building program was only outmatched by the Americans. And both of these nations navies took up considerably less percentage of their respective national budgets than Japan's. So the treaty limitations seem to be more in line with if the select nations could actually build in terms of financial/industrial strength and political will than racism or any other prejudices.
Finally, Japan did make concessions, but so did UK and US, they signed into agreements such as no further fortifying and construction of naval bases in Pacific with a few exceptions like Australia which gives Japan much more advantages, Article XI targets Britain in all but explicit name calling, Article XVIII pertains to US and UK's specific circumstances as well. The general circumstance as well, Britain has to manage the largest empire in history with interests even far more reaching, the Americans have to protect two oceans with a vast coastline and also extensive interests in Latin America while Japan has to look after a relatively small corner of the world in east Asia.
It doesn't seem remotely unfair, am I just being a pedantic asshole?
You're generally right in your assessment that the Japanese got a fairly good deal out of the Washington Naval Conference. It set artificial limits on what they could expect to fight with respect to their most likely adversary - the United States Navy - both in terms of quantity and quality, and the prohibition on fortifying bases in the Pacific clearly favored the Japanese more than the other signatories.
Perhaps just as importantly, it forced limits on the IJN's expansion that prevented them from building beyond the limits of what their economy could support. For example, while the Japanese were spending about 20% of their GDP on military spending by 1938, the US, up until entering WW2, was running closer to 1.5 to 2%.
But rational hindsight with all the information easily at hand doesn't always work when looking back on historical figures. The biggest issue at the face of it was how the Conference undermined what had been a core tenet of Japanese naval strategy - a fleet that was 70% that of the US Navy. Based on some statistical assumptions relating to losses, the IJN had determined that a fleet 70% the size of the USN would be able to defeat the US Pacific Fleet in a decisive engagement while retaining enough of its fighting strength to defeat the US Atlantic Fleet that would likely be moved to the region after the first engagement. This strategic concept, which reached its peak in the 8-8 fleet, was undermined not just in the tonnage allotments (giving the IJN only 60% the tonnage of the USN), but in technical terms as well, as it made it more difficult for the IJN to procure (on paper) superior capital ships to those of the USN.
Japanese naval leadership, keeping this in mind, were aggressive in their outlook and rightfully so felt that the Washington Naval Conference undermined their entire strategic vision. They would feed off of a jingoistic domestic media apparatus that would be the staple of Japan's long leadup to war, and, while calmer parties within the Japanese government (and even the Navy itself) would recognize that the Conference was a blessing for Japan's economy, jingoism would make it easy to spin this as another thing to add to the decades-long list of (valid) slights to Japan's honor done by the West.
Oddly enough, the reality of naval construction through the Treaty Era meant that Japan's 70% goal was actually reached. The IJN aggressively pursued its building policy, even going as far as fudging numbers and building treaty-circumventing ships like Ryujo, while the US had done only modest upgrades, owing to a resurgence in isolationist sentiment and the onset of the Great Depression. Because of that, for much of the Treaty Era the IJN actually was around the 70% ratio it had so desperately wanted with the USN.
Unfortunately for the IJN, that 70% ratio wouldn't last very long. Once the USN began a new wave of construction as tensions brewed in the leadup to WW2, the extremely poor strategic position of Japan became apparent - and along with it, the benefits of the Washington Naval Treaty.
There's already a good answer, but I would add that when the Washington Naval Treaty was born, there was a strong pre-existing narrative in Japan (among nationalists in particular) that Japan was always getting screwed over by the West through negotiations.
This narrative started with the Ansei "unequal treaties" signed by the Japanese with the US and European powers in the 1850's when Japan was forcibly "opened up," very much on Western terms.
Then in 1905 Japan agreed to the Port Arthur treaty, ending the Russo-Japanese War. The peace treaty was very unpopular in Japan, because most of the Japanese public did not realize that, despite their military successes during the war, Japan was dangerously overextended and really couldn't afford to continue the war. The peace treaty was met with rioting in Japan and it eventually brought down the government.
The peace negotiations in WW I added to this feeling. The Japanese pushed during the Paris Peace Conference of 1919 the concept of racial equality for League of Nations members. Part of why they wanted it was directly related to prior experience negotiating treaties with the West. The measure ultimately was not adopted (the US and UK in particular were opposed to it), and this further cemented in Japanese minds that they were getting the short end of the stick when negotiating with the West.
So while I think there are objective arguments as to why the Washington Naval Treaty was actually not unfair to the Japanese, the fact that Japan on the surface was more limited than the US and UK once again fed the (somewhat justified) narrative that the West was not negotiating fairly with Japan and was not treating Japan as an equal.