Show that boring boolean circuit belongs to NP-complete class
The problem is PP-hard. This means that unless the polynomial hierarchy collapses to NP, then deciding whether a circuit is boring is not in NP (and consequently is not NP-complete). The collapse follows from the fact that PP is closed under complement, so $\mathsf{PP=NP}$ implies $\mathsf{NP=coNP}$. We now show that deciding whether or not a circuit is boring is PP-hard.

Suppose $L\in PP$, then there exists a probabilistic polynomial Turing machine $M$ such that $x\in L \iff \Pr\left[M(x)=L(x)\right]&gt;\frac{2}{3}$. Now consider the machine $M'$ which tosses two coins, if both output "heads" accept, otherwise execute $M$. If $x\in L$ then $M'$ accepts $x$ with probability $\frac{1}{4}+\frac{3}{4}\Pr\left[M(x)=L(x)\right]&gt;\frac{3}{4}$. Now, let $C_x$ be the corresponding circuit for $M'$ on input $x$ (its inputs are the coins for $M'$), then $x\mapsto C_x$ is a reduction from $L$ to your problem, since $x\in L\iff C_x \text{ is boring}$. To see why, note that if $x\in L$ then $C_x$ accepts more than $\frac{3}{4}$ of the possible inputs, and if $x\notin L$ then $C_x$ accepts $\frac{1}{4}\le t\le \frac{3}{4}$ of the possible inputs.

In fact, your problem is PP-complete. To see why this language lies in PP, Let $L$ be the language of circuits who accept more than a $\frac{3}{4}$ fraction of their input, and $L'$ be the language of circuits who reject more than a $\frac{3}{4}$ fraction of their inputs. Clearly $L,L'\in PP$, and since PP is closed under union, deciding whether a circuit is boring, i.e is in $L\cup L'$, is in PP. Note that both here and in the above we used the fact that the constant in the definition of PP can be changed to any rational number.

If we want to avoid using closure properties of PP, then an argument along the following lines should work. Consider the machine $M$, which upon given an $n$-input circuit $C$, approximates the acceptance probability of $C$ by evaluating it on different uniformly distributed random length $n$ strings. Let $\hat{p}$ denote our evaluation. If $\hat{p}\ge\frac{1}{2}$, then $M$ evaluates $C$ on a random string and accept iff $C$ accepts, otherwise $M$ accepts iff $C$ rejects.
Let $E$ denote the event that $|p-\hat{p}|\le \frac{1}{4}$, where $p$ is the real acceptance probability of $C$. By evaluating $C$ on polynomially many strings, we can make sure that $\Pr\left[E\right]\ge 1-\frac{1}{2^{2n}}$. If $C$ is boring, then conditioned on $E$, $M$ obtained the right evaluation of the majority of $C$ and accepts with probability $&gt;\frac{3}{4}$. Thus, if $C$ is boring $M$ accepts with probability $&gt;\frac{3}{4}\left(1-\frac{1}{2^{2n}}\right)$. If $C$ isn't boring, then $C$ outputs either $0$ or $1$ with probability of at most $\frac{3}{4}-\frac{1}{2^n}$. Thus, regardless of our evaluation $\hat{p}$, $M$ accepts $C$ with probability of at most $\frac{3}{4}-\frac{1}{2^n}&lt;\frac{3}{4}\left(1-\frac{1}{2^{2n}}\right)$, which puts our language in PP.