How is Belinda representative of eighteenth-century women?

Belinda represents not so much eighteenth-century women as she does an ideal of glamor and womanhood that was typical of the upper-class mentality of the time. In this sense, she's not much different from celebrities of our own age who are celebrated primarily for beauty and style, rather than the "deeper" qualities people see and admire in others.
Pope views the attitudes of his time with irony and, I would argue, with a large degree of empathy for women. It's interesting that he includes the young men attending the party in his focus upon appearances and the less-than-"profound" qualities of people, as he does Belinda and other women:

Fair nymphs, and well-dress'd youths around her shone,But ev'ry eye was fix'd on her alone.

If superficiality is attributed to Belinda, Pope makes the same point about her admirer:

For this, ere Phoebus rose, he had implor'dPropitious Heav'n, and every power adored,But chiefly love—to love an altar built,Of twelve vast French romances, neatly gilt.There lay three garters, half a pair of gloves;And all the former trophies of his loves.

The Baron, then, is a ladies' man, having had previous conquests and now intending to add Belinda to his list. Though one might think its satiric object is the heroic poetry of Homer and Virgil, the mock-heroic diction is actually spoofing the manners of Pope's own age and the trivialities that are made into important things in the upper-class mentality. Belinda, in her vanity, is no better or worse than the whole culture that surrounds her. Also, the vanity itself in her case might simply be a facade:

Her lively looks a sprightly mind disclose,Quick as her eyes, and as unfix'd as those.

Belinda is not vapid or empty-headed, as some readers might assume. The following lines can be understood in multiple ways:

Yet graceful ease, and sweetness void of pride,Might hide her faults, if belles had faults to hide:If to her share some female errors fall,Look on her face, and you'll forget 'em all.

My interpretation is that Pope is parodying his own era's superficial view of women and the belief that young women must be faultless and that their good looks are the only factor canceling whatever faults they do, in fact, have. Belinda is therefore a symbol of the unfair expectations men have of women in Pope's time—and by extension, throughout all time.
One might say I'm stretching things to attribute feminist attitudes to Pope. However, Pope himself was a kind of outsider in society, for several reasons, and probably had more understanding of gender roles and more sympathy for the position of women in the world than most men of his time did. He was hunchbacked and a little person. As a Roman Catholic, he was at an additional social disadvantage. Yet his talent and his intellectual powers gave him an entry into the elite world, from which he was able to view, and ironically report on, the unfair and stereotyped attitudes of the eighteenth-century world, which unfortunately have persisted in some ways up through our own time.