Using 'simper' suggests a negative view of someone's smile as insincere or silly. When he complimented her, she just simpered, not knowing how to respond.
Pro Tip 2/3
Insincere Flattery
'Simper' often implies flattery that is not genuine or is overdone. He simpered at his boss, hoping for a promotion.
Pro Tip 3/3
Self-Conscious
The verb 'simper' can indicate a person is aware and possibly embarrassed about being watched or noticed. She simpered during her presentation, aware of all eyes on her.