My guess is that she was killed in fall or perhaps spring, when thermal underwear would be reasonable but a coat might not be necessary. I'd have to check the weather for the 1993-4 season, but Boston winters are usually pretty darn cold, even without snow.
Was she wearing long underwear bottoms, or just a top? Thermal tops under t-shirts are pretty common year round, except in summer.
Re the t-shirt: I was actually thinking that it came from a discount store (like K-Mart or similar). It reminds me of shirts you'd find thereunbranded, colorful, just made from Fruit of the Loom.
My theory is that she was walking or hanging out in the park with an acquaintance, possibly smoking (I think a lighter or cigs were found nearby?), possibly doing drugs or doing a deal (not to malign the victim at all, but that's fairly common behavior in city parks). Maybe she was transient, maybe just estranged from family. Her "friend" killed her, took any money, ID, or drugs if she had them, and left.
I don't live very far from Stony Brook; I'm happy to go over and take some pics or do other research if it helps.