If your allergy is something that happens just on your lips, like an allergy to toothpaste or to food and drink touching your lips, or to airborne cat dander or dust mites, than Benadryl cream on your lips may actually help you.
If your allergy is something that happens inside your intestines, like allergy to Gluten, milk, wheat, grains, soy, corn, nuts, tomatoes, etc ... than Benadryl cream on your lips may actually be of no help to you.
You may as well be a combination of both, an in that case it may help to a degree, but not 100%.