In a large Dutch oven over medium-low heat, heat oil. Add onion and cook, stirring occasionally, until softened, about 10 minutes. Increase heat to medium-high, add taco seasoning, and cook, stirring, until fragrant, about 1 minute. Cook beef, breaking up with a spoon, until no longer pink, about 5 minutes; season with salt.
Add broth, tomatoes, beans, and corn and bring to a boil. Reduce heat to medium, bring to a simmer, and cook, stirring occasionally, until slightly reduced and thickened, about 10 minutes.
Meanwhile, in a small bowl, mix sour cream and Tajín until combined.
Divide soup among bowls. Top with cilantro, a dollop of sour cream-Tajín, and cheddar. Serve with chips alongside.
