should the excess fat be trimmed off the thighs?

Unless your chicken had serious thunder thighs in life, and gobs of fat attached, I would not think trimming would be necessary. You are going to be grinding in the skin, which contains a good bit of fat, so it's not like these are low fat diet burgers. Hopefully extraneous fat would render off in the cooking process. If there are large amounts and you want to remove it, go ahead and do it. Or not. It's not like there are chicken burger police lurking in the shadows waiting to come haul you away.