"Bonus" can't be negative
Player's Handbook makes a distinction between "a bonus" and "a penalty":
A class feature, a spell, a particular circumstance, or some other effect might give a bonus or penalty to the check
"Bonus" is a positive modifier:
His attack bonus is his Strength modifier (+3) plus his proficiency bonus (+2), for a total of +5.
"Penalty" is a negative modifier:
The target takes a -4 penalty to all attack rolls
When a modifier is meant to be positive or negative, the PH doesn't say you "gain a bonus", or "add bonus", it just says you "add modifier". Here's an example where always positive modifier is called "bonus", while a potential negative one is called simply "modifier" (emphasis mine):
When you make an attack with a weapon, you roll a d20 and add your proficiency bonus (but only if you are proficient with the weapon) and the appropriate ability modifier.
The Elegant Courtier explicitly says you get a bonus:
you gain a bonus to the check equal to your Wisdom modifier
If your WIS modifier is negative, it can't be a bonus. This complies with the narrative part:
your discipline and attention to detail allow you to excel in social situations