A small pimple on a cat's neck may be caused by mosquito bites, swollen lymph nodes, or subtonitis caused by injections. If the cat has just received an injection and a small pimple appears, you can use a hot towel to apply heat to the cat. Yes; if it is not caused by injections, it is best to take the cat to the pet hospital for examination to see what is causing it, and then treat it after confirming the cause.