It's a joke (for the most part)
Firstly, the "May Contain" is a common warning in many countries by manufacturers to indicate that not only might the product actually contain allergen items but also that the allergen may have entered the product accidentally ... usually as it was manufactured in the same plant where the allergen was being used.
Manufacturers often use phrases such as 'may contain' to show that there could be small amounts of an allergen for example milk, egg, nuts etc. in a food product because it has entered the product accidentally during the production process.
It's not a legal requirement to say on the label that a food might accidentally contain small amounts of an allergen, but many manufacturers label their products in this way to warn their customers of this risk.
But secondly, and perhaps more importantly, nuts is used as a double entendre to imply testicles since Wallace is naked in the box....or perhaps that Wallace is just a little crazy (nuts).