This is only a problem if you naively undersample the normal maps in your fragment shader. No AA algorithm can can generate a high quality result from undersampled lighting calculations.

For normal maps specifically, you can preserve lost information from downsampled normal maps as roughness: https://media.steampowered.com/apps/valve/2015/Alex_Vlachos_...

MSAA also doesn't require you to have only one color sample for each pixel but this can be varied per draw call (at least for desktop GPUs) - effectively allowing you to supersample some objects when needed.