Why is Mexico not a democracy?

Answer 1

China is not considered a democracy because it is a communist country; as such, it has one recognized party and although "elections" take place, it is only among different candidates of the same party.

For Mexico, it is a federal republic just like the United States; the problem lies in the fragility of its institutions and how corruptible the election process can become. For example, "vote purchasing" is still performed on less advanced regions within Mexico. Then, it is not a true democracy because the will of the people is not always respected.

Answer 2

It is, in the sense that it shares the same political system of countries like the United States, France or Germany (i.e. Representative Republic).

It isn't in the sense that Mexico is a developing country with weak, corruptible institutions, preventing the proper execution of the law.