She . . . . . . . . be very silly at times.

Both 'can' and 'could' are modal verbs that refer to 'a possibility', 'ability' or 'capacity'. 'Can' refers to a general truth or something that has a strong sense of possibility. 'Could' refers to something that has a weak possibility, or something that might happen, but is not necessarily a general truth. Hence, 'could' is the suitable answer here.