In some cases, Cosmo's stupidity rubs off the wrong way on Timmy, and he is usually the one causing Timmy to

Cosmo and Timmy

[1]. Cosmo and Timmy make a bad wish when Wanda is advising against it. Cosmo and Timmy share more of a buddy relationship rather than a father-son relationship. Despite falling for Cosmo's ideas, Timmy is usually well aware of how intelligent Cosmo really is, but if it sounds fun enough Timmy will go with it anyways.