Is a lack of ambiguity necessary for clear thinking? Are there times when it's better to be ambiguous?

No. Yes.

I interpret "ambiguity" as "having multiple meanings in the given context". It can be a bad thing or a good thing. The "optimal level of ambiguity", I think, is very much situation-dependent and I'm not optimistic about formulating any general rules for that.

Off the top of my head I can come up with a couple of way ambiguity is useful. One is when you are thinking at several levels simultaneously. In such a case ambiguity can provide vital linkages between levels helping to keep them in sync. Two is when you use ambiguity to postpone choosing a path for a bit. Here it's similar to a wavefunction which you might find convenient to hold in an uncollapsed state for while and then collapse it (= resolve the ambiguity) at the right moment.