The first problem is that Noah lays upon his people what Mormon considers a heavy tax. A 20% tax on income doesn’t seem excessive for modern taxpayers in the United States, and even less excessive compared to other modern states. For Mormon, it was not simply that the taxes were levied, but that they were levied in support of a system supporting social inequality. The taxes were not directly beneficial to all, but were to support the socially inequitable lifestyles of the elite.
That is the reason that Mormon mentions that this tax was to support himself and his wives and concubines. Contrast this use of public funds to king Benjamin’s declaration that he had not sought riches from his people (Mosiah 2:12). The fundamental issue is the use of the funds, not the level of taxation. In the case of king Noah, taxation was not used to improve all lives, but to specifically enhance the social segregation into at least an upper and a lower class.