Single responsibility principle ​

public function getFullNameAttribute(): string
{
    if (auth()->user() && auth()->user()->hasRole('client') && auth()->user()->isVerified()) {
        return 'Mr. ' . $this->first_name . ' ' . $this->middle_name . ' ' . $this->last_name;
    } else {
        return $this->first_name[0] . '. ' . $this->last_name;
    }
}

public function getFullNameAttribute(): string
{
    return $this->isVerifiedClient() ? $this->getFullNameLong() : $this->getFullNameShort();
}

public function isVerifiedClient(): bool
{
    return auth()->user() && auth()->user()->hasRole('client') && auth()->user()->isVerified();
}

public function getFullNameLong(): string
{
    return 'Mr. ' . $this->first_name . ' ' . $this->middle_name . ' ' . $this->last_name;
}

public function getFullNameShort(): string
{
    return $this->first_name[0] . '. ' . $this->last_name;
}