There's nothing wrong with ketchup, Tara, as long as your toddler is also eating the foods underneath! Some children will actually try new food as long as it has a familiar taste (in this case, the ketchup). Most 2-year-olds are very resistant to change, so ride this one out. As the taste buds mature, you will probably see a change in this habit.
By the way, my 18-year-old just went through a similar phrase with Cajun seasoning. It only lasted a few weeks!
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.