The poem is called "Upon Julia's Clothes" largely because (prepare yourself) it is about how the speaker feels when he looks at Julia's clothes. We know, we just blew your mind, right?
Sure, the title seems simple enough. But there's a bit more to it. The poem also seems to be about finding different ways to describe Julia's clothes. From this perspective, then, "Upon Julia's Clothes" means something like "here's a poem about different ways of describing Julia's clothes."
Of course it's not really about her clothes, per se. It's all about how those clothes make our speaker feel. And they make him feel really, really good. He swoons, he's taken by them, he's plainly plumb in love with this woman's style.