For computing the NDVI I want to use USGS Landsat 8 Collection 2 Tier 1 TOA Reflectance
LANDSAT/LC08/C02/T1_TOA does it need cloud masking or any other form of ''cleaning'' before I can use it?
The Landsat 8 Collection 2 encompasses Level-1 (top of atmosphere reflectance) and Level-2 (atmospherically corrected surface reflectance) products. Since Landsat 8 Collection 2 level-2 correspond to surface reflectance and;
corrected for the temporally, spatially and spectrally varying scattering and absorbing effects of atmospheric gases, aerosols, and water vapor
it should be used to generate NDVI.
However, you need to apply scaling factor i.e.,
0.0000275 + -0.2 for Level-2 before calculating NDVI. You should mask pixels which are contaminated by clouds and shadows as well.
Here, I would recommend a worth reading article "A survival guide to Landsat preprocessing" for further enlightenment.
More information about Landsat Collection 2 Level-2 Science Products can be accessed from here.