The manga is talking about a different Mardi Gras. There's a translator's note saying its a Mardi Gras in Australia. http://www.mangago.me/read-manga/uchi_no_darling_gaikokujin/tr/v01/c003/1.html/pg-9/
I lived in New Orleans for 4 years of college. If I had not read the translator's note regarding the Mardi Gras in Australia, I'd have been confused too. Mardi Gras in New Orleans, all of Louisiana and parts of Texas, Mississippi, and Arkansas is actually a Christian thing. It is the Tuesday before Ash Wednesday, the beginning of Lent. Since Christians abstain from things that are considered indulgent for Lent, Mardi Gras or Fat Tuesday is a day of indulgences before abstinence. Mardi is French for Tuesday and Gras is French for fat or large. Hence, Fat Tuesday. Louisianans have stretched this day out to a week of parades and parties that culminate on Fat Tuesday. Though there are parades for, and parade participants who are from, the LGBT community, Mardi Gras is not solely and LGBT event.
To the original commentator, I'm sure you are aware of all of what I have written here. I wrote all of this for the benefit of others who might read it, but your comment gave me the opportunity to clarify your statement. I hope I caused you no insult or offense by this clarification. I meant no ill will. Thank you for the opportunity.
In the U.S. Mardi Gras isn't a lgbt holiday, and I know, I'm from Galveston which is like Texas' New Orleans