Optimization of Church-encoded booleans in System F
Without further assumptions you cannot perform the optimization because it is unsound in general. For example, we could interpret System F in a model that has fixed point operators, in which case there will be elements of Bool that are different from false and true (namely "undefined" ⊥). In such a model, dumb will not be equal to \ b . false because dumb ⊥ = ⊥ but (\b . false) ⊥ = false.

In your question you assumed that we know what it means to "optimize" a function. If we leave aside questions about efficiency, we still have to answer the question what it means for two closed terms in System F to be equal. There are several possibilities:


closed terms $M$ and $N$ are equal if they reduce to the same normal form,
closed terms $M$ and $N$ are equal if they are equal in some preferred model,
closed terms $M$ and $N$ are equal if they are observationally equivalent,
closed terms $M$ and $N$ are equal if they are extensionally equivalent (they give extensionally equivalent results when applied to extensionally equivalent arguments).


Parametricity, which was mentioned in the comments, may be helpful with some of the above. But it's difficult to discuss "optimization" until we know what counts as "equal", or else we cannot tell which optimizations are valid.