For this specific case you can do ->

if (filteredMnth === 'mnth') {
  return new Date(exp?.date).getMonth() <= 6;
}

A nice pattern for this, is to create an Array of valid or accepted values and use the Array.prototype.includes to check for one from the user input:

const validValues = [ 0, 1, 2 ];
const input = 2;

validValues.includes(input);
// => true

const input2 = 3;

validValues.includes(input2);
//=> false

As @Robby Cornelissen already mentioned in the comment, in your case it really does not make any sense, but I'm including this pattern here to answer a more generic version of your question.

Since you are having multiple values you can make use of List and includes method as below

const validMonths = ["1", "2", ...]
const monthToCheck = new Date(exp?.date).getMonth().toString()
if(validMonths.includes(monthToCheck)){
    //Evaluates true if value exist
}