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Mrs. Cartman will contact you once you have enough followers in Coonstagram or after completing The Hundred Hands of Chaos. (Not really sure about the ballpark figure but this sidequest triggered for me after the latter main quest)
Head over to Cartman’s house and talk to his mom by the kitchen. She will give you her business cards which you have to deliver to prospective “clients”. Head to Skeeter’s Bar and hand out the business cards to the three specific gentlemen. (Use inspection mode to identify them) After handing them over, report back to Mrs. Cartman to complete the quest. You’ll receive Ms. Cartman’s Double Stuffed Brownies.
The brownies you received is a main component of a special burrito that will unlock the optional TimeFart: Summon. Follow this page to learn how to unlock this power.