good or bad idea update on a 5 to the new IOS?

The 5C should do nothing. It should not try to update, since it can't, and the update would not be pushed to it at all.

If its getting an update notification, then its likely not a 5c, but something else that can support the update and as such is getting the notification.

The iPhone 5c has a plastic pastel colored back plate, unlike the aluminum back plates of the iPhone 5, 5s, and SE.

An iPhone 5c cannot be updated to iOS 11.

An iPhone 5 (no letters) can also not be updated to iOS 11.

To update to iOS 11 you require an iPhone 5s(Five S) or later.

An iPhone SE (no numbers), can be updated, and should not be slowed down, or have any negative impact from the update.

iOS 11 is compatible with these devices - Apple Support

The 5c cannot be updated beyond iOS 10.3.3. There is no 5se. You may mean the 5s or the iPhone SE. I have a 5s that runs very well with iOS 11.1 and the iPhone SE is a much newer model and should have no problems.

It won't ask to update or try to update. It knows that it is up to date as far as that model is concerned.