Are There Always More Conjugacy Classes in the Kernel of a Morphism to $\Bbb Z_2$ than Not?

This is true!

Let $\phi : G \to \mathbb{Z}/2$ be a group homomorphism, where $G$ is finite. By composing with $n \mapsto (-1)^n : \mathbb{Z}/2 \to \mathbb{C} \setminus \{0\}$, we produce a $1$-dimensional character of $G$. All the entries in the corresponding row of the character table will be $\pm 1$, and we want to show that the sum of the entries in this row is non-negative. But the sum of the entries of any row of the character table of any finite group is always a non-negative integer! A proof follows.

Proof. Let $\psi$ be the permutation character of $G$ acting on itself by conjugation. By thinking about the permutation matrices, we see that $\psi(g) = \lvert C(g) \rvert$ (the order of the centralizer). Since $\psi$ is a character, $\langle \chi, \psi \rangle$ is a non-negative integer for all irreducible characters $\chi$. But

$$\langle \chi, \psi \rangle = \frac{1}{\lvert G \rvert} \sum_{g \in G} \chi(g) \overline{\psi(g)} = \frac{1}{\lvert G \rvert} \sum_{g \in G} \lvert C(g) \rvert \chi(g)$$

is precisely the sum of the entries of the $\chi$-row of the character table.

The following also holds

Claim Let $f : G \to \mathbb{Z}/3$ be a homomorphism. Then $\ker f$ contains at least $1/3$rd of the conjugacy classes of $G$.

Proof. By composing with $n \mapsto \zeta^n : \mathbb{Z}/3 \to \mathbb{C} \setminus \{0\}$, where $\zeta = e^{2 \pi i / 3}$, we get a $1$-dimensional character $\chi$ of $G$. The entries in the corresponding row of the character table are all $1$, $\zeta$, or $\overline{\zeta}$, and these sum to some $m \in \mathbb{N}$.

So, we have $a + b \zeta + c \overline{\zeta} = m$ for some $a, b, c \in \mathbb{N}$, where $a$ is the number of conjugacy classes of $G$ which are contained in $\ker f$ and $a+b+c$ is the total number of conjugacy classes of $G$. By equating imaginary parts, we get that $b = c$, so $a - b = a + b(\zeta + \overline{\zeta}) = m \geq 0$. This means that $a \geq b$, so $$a+b+c = a + 2b \leq 3a,$$ as desired.