To me, "either" seems to mean that there's a choice, for example:
"You can have either the ice cream or the cookie," meaning not both.
So in an example where it does mean both, like yours, I would not include the either.
Another option is rewording it to use "neither...nor", like this:
"I like neither the cap nor the shirt," though "neither...nor" isn't used as often in everyday speech.