You mean like their shoes, the shirt they're wearing, that kind of thing? Because the eye patch is only unusual to you, the person with the eye patch puts it on every morning. Hell, they might have patches in different colors.
You can ask, but keep in mind that asking only benefits you. Because I'm sure the person wearing the patch has long ago gotten their fill of answering that question, and doing so is no longer of benefit to them.
So to speak personally, I don't ask. It's obvious that there is some malfunction in one of their eyes, I don't need to ask because that's all I need to know. Asking for more detail, well, I just met the person. Why would I think I can now delve into their personal details just because they're wearing an eye patch? More so, I don't ask because I'll betcha they are sick...and...tired of answering to random strangers who, truth be told, don't give a shit about the person and just want an answer to their curiosity.