I think it's one of those things that Harry knows, but he hasn't "got it"... I mean, he clearly had enough evidence from Snape's memory to know that Lily Potter was once Lily Evans. But I don't think he's put it together (he heard it, but was so focused on what he was seeing at the time, he didn't pay attention to what they called her)... Even if he does know that her maiden name was Evans, I don't think he's put the Evans-Evans thing together... Mainly because he is working from the "knowledge" that he has no other relatives. If you "know" you have no relatives, you don't stop and ponder if the same name thing is a coincidence. It's kinda like, in the Mirror of Erised, Harry's greatest desire is to see his dead relatives, since, supposedly, all his relatives are dead except the Dursleys.
That, along with the fact that Evans is really a fairly common name (common in Harry's world) makes me think that he wouldn't put a lot of thought into Mark Evans. Really, the only reason
we question it is because it's against the nature of J.K. Rowling to put even a small character in there with the same name as Lily without there being something involved with the story. But if we look at it like Harry's looking at it, unless our mother had a VERY odd maiden name, we wouldn't assume or even usually think about the possibility that they're related, especially if we don't think we have living relatives.
My mom's maiden name is Jones. Any time I meet someone with the maiden name Jones, it doesn't cross my mind to ask if they're from the same little town, or if they know someone or are related to someone in Hallsville. A kid my sister went to school with had the last name (and I can't spell it, I'll have to sound it out) hab-bib-bli-ah-hi-an. It would be fairly safe, if he met someone with the same name, to ask if they are from or know someone from the same general area, and chances are more with him that they would be related somehow.
Evans is a more common name, so it's not that odd (to Harry) that there's a kid down the street with the same name as his mom's maiden name. Even more reason to think this is the fact that he would probably assume that if Lily had a relative on Privet Drive (or the surrounding area) then Petunia would have a relative on Privet Drive (or the surrounding area), and since he's never met this relative, he would assume there isn't one.