I think both ways are possible in English, but should I say
This is a corollary to Theorem 1.2
This is a corollary of Theorem 1.2
In contemporary U.S. English, "corollary to X" would be (slightly intellectual-sounding) non-math usage, so also in-principle-legit in math, while the "corollary of X" is specific U.S.-math-English usage. The only slight bias might be to use "corollary of" because it sounds more typically math-y... ?