Allowing yourself to be critical at the same time you are being creative is like trying to get hot and cold water from one faucet at the same time. Ideas are not hot enough, criticism is not cold enough.
Results are tepid.
The wilder the ideas, the better. Even offbeat, impractical suggestions may trigger in others practical suggestions which may not otherwise occur to them. It is easier to pare down a long list of ideas than to puff up a short one.