Should I add some leftover pastry cream to the pumpkin mixture for this galette?

How exactly would you be adding the pastry cream? I'd be concerned about it possibly separating and also making the fruit look muddy.

I have made this galette several times and it is delicious as-is, but the pastry cream sounds like a delicious addition, it seems like it would "custard up" the pumpkin part - how could that be bad???

If your aim is to modify the filling, yes, but reduce the amounts of pumpkin and/or apple to keep total volume the same. If the aim is to use up delicious pastry cream, save it & plate as swirls or dots on the dessert plates, then a piece of galette on top.