What a lovely awful Frenchman.
Like her or not, Nyaruko is cute.
And I’ve long determined that her human form is her most stable state.
If it weren’t, she’s revert into whatever shape were natural for her species when under extreme stress, unconscious, or not in control of her body, all of which have happened in the anime.
And in any case where this has happened, she either turns human or remains human. And when stripped of all her power and on death’s door she’s still human, so therefore the eldritch horror form is something that requires a great deal of effort to assume.
Thus we can conclude that Nyaruko is not in fact a terrible Frenchman, and is just the cute girl we’ve been fooled into thinking she is.