Well, like, is it technically incorrect grammar?
In English, we put the plural status as part of the noun, but the Japanese language uses things like counters and the noun 日本人 does not actually specify whether you are referring to one or more Japanese people, as “neko” does not specify the number of cats, and so on. Japanese also doesn’t have an a/an/the attached to the noun.
So, if we adopt either a descriptivist mindset or some sort of cultural prescriptivist mindset, it could be argued that Japanese/Japanese is valid just as ninja/ninja and German/Germans.
That also brings up that there is no simple plural form such as “Germans”, and I don’t think anyone anywhere will approve of “Japaneses”. (Wow, “Japaneses” sounds really racist.)
So I guess it’s down to whether the listeners/readers socially approve of it, much like “Brits” is okay, but “Japs” and “Nps” were both part of pretty damned racist WW2 propaganda and are thus permanently prohibited, even though all three are just shortenings of national names.