The anticholinergic compounds (atropine and others) in deadly nightshade would cause dilation of the pupils, drawing attention to the eyes and also simulating arousal/excitement. Systemic absorption might also cause some flushing and warmth of the skin, also attracting attention. Doctors still use compounds like this in the eye today in order to dilate the pupil to look inside the eye. But not using completely uncontrolled plant extracts avoid the risk of systemic toxicity.

What I don't know, and couldn't find anywhere, is a historical source for people actually using nightshade/atropine in the eyes for cosmetic effect. I can find assertions that it was done by the Romans and by Italians in the Renaissance, but no actual sources describing its use. I'd be interested in seeing these if anyone else has any information.