We dont need to iterate the entire string we can return false as soon as we first find the mismatched braces. This means we dont need to check the length of the stack at the end. Please see my solution below:

public bool isValid(string s) {
        
        if(s.Length==1) // One char string for sure would not be balanced
            return false;
            
        Dictionary<char,char> closingOpeningBracesPairMap = new Dictionary<char,char>();
        closingOpeningBracesPairMap.Add(')','(');
        closingOpeningBracesPairMap.Add(']','[');
        closingOpeningBracesPairMap.Add('}','{');

        Stack<char> stack = new Stack<char>();
      
        for(int i = 0; i < s.Length; i++)
        {
            if(IsOpeningBrace(s[i]))
            {
                stack.Push(s[i]);
            }
            else
            {
                var poppedElement = stack.Pop();
                if(poppedElement != closingOpeningBracesPairMap[s[i]])
                    return false;
            }

        }

        return true;
    }

    private bool IsOpeningBrace(char brace)
    {
        if(brace == '(' || brace =='[' || brace == '{')
            return true;
        else
            return false;
    }