It is, at least sometimes. Books specifically on the Taiping may not often play this angle up, for sure: it's not heavily expounded on in Jonathan Spence's God's Chinese Son, receives only a passing mention in Stephen Platt's Autumn in the Heavenly Kingdom, and I don't recall it coming up at all in Tobie Meyer-Fong's What Remains. However, studies specifically of ethnicity and ethnic relations in the Qing period do frequently bring up the idea of the Taiping as being uniquely, for lack of a better word, racist. In particular, Pamela Crossley, Mark C. Elliott and Edward J. M. Rhoads have grappled – though to varying extents – with Taiping ethnic rhetoric, albeit as part of broader discussions of ethnicity in the Qing era – see Orphan Warriors and A Translucent Mirror by Crossley, 'Bannerman and Townsman: Ethnic Tension in Nineteenth-century Jiangnan' by Elliott, and Manchus and Han by Rhoads.

As for why the ethnic angle of the Taiping hasn't been brought up much in specific works on the Taiping, put simply the New Qing approach that emphasises such angles is relatively new, and arguably the idea of Manchuness only really got to be refined around the turn of the millennium. Since then I'm aware of only 4 books on the Taiping in English, of which two focus on religion, one on the military-political narrative of 1859-64 and one on the general devastation of the Jiangnan region and people's response to it. While there's been plenty of opportunity for a more specialised analysis, it just doesn't seem to have been done yet.

Another factor may well be that the Taiping never really got to the stage where genocide was a consistent part of their policy. Nanjing and Hangzhou were the only two cities with significant Banner garrisons that were captured and their garrisons massacred, and while the largest concentrations in the Jiangnan region, they constituted 'only' around 5000 total households, or in other words maybe a twentieth at most of the total Banner/Manchu population. In numerical terms this was not insubstantial by any account, but it was nonetheless not enough to constitute a major demographic shift on the national level. In many ways we should be thankful the Taiping did not get further. By contrast, a third of the Manchu population lived in areas directly affected by the 1911 Revolution, and of course their future hung very much in the balance during the negotiations over the new republican state. Consequently, you have interest from scholars like Edward J. M. Rhaods who had studied the revolution in the past, taking new approaches that account for the complexities of ethnicity in the Late Imperial period.