Why don't Marty's parents ever mention how similar he looks to the Marty of 1955?

There is a definitive answer to one part of this question:

Is it likely that the producers simply overlooked this?

No. It was intentionally removed from the original draft. (Obviously this version is fairly different)

Here's the original ending of the script:

INSERT – CLIPPING A story with the headline, “Police Quell Near Riot At School Dance,” along with a photo of the dance that shows Marty on stage!

GEORGE stares at the photo, then shakes his head.

GEORGE Nah. Couldn’t be.

Even if they would notice, they would never accept it as a truth, because it is much against a normal understanding of the world.

It would be probably like:

"Ever notice how much Marty looks like that one guy I nearly fell in love with?"

"Yeah, funny that".

And thats it :). They could never imagine/reason, that Marty did time travel. So it is not even a possibility and therefore would not seem to be strange.

Remember that cat your parents took in for one week when you were a child? Isn't it strange that it looks exactly like the kitten you've been raising down to the last stripe?

You don't remember what that cat looked like?

I don't remember my childhood best friend's face very well and I knew him for years. Had I met him and known him for only a week I doubt I would even remember the details.

This isn't a plot hole.

Suppose the parents did notice the similarity. What would the chances be that they noticed exactly in the '85 time frames the movie shows? Nil.

It doesn't matter

His parents never mention the similarity in the movie because:

1. Before the 1955 part of the movie, it would be somewhat of a spoiler.

2. After the 1955 part of the movie, it might have worked, but it would have been "cute dialog of convenience" and not really realistic there.

Ultimately, if you accept the underlying premise of Back to the Future which is that if key events stay more or less the same, the future will be unchanged, then you have to accept that whether his parents saw the similarity or not is irrelevant. This is good news. You can either choose to believe they never noticed a similarity, or you can choose to believe that at some point in Marty's alternate youth, his parents showed him a picture of this guy he bears an uncanny resemblance to, and none of them including alternate past Marty had any idea it actually was him yet, and then they went about the rest of their lives normally. Whether either theory is correct is irrelevant, because according to the rules of the BTTF universe, it doesn't actually change anything.

Who is to say they DIDN'T notice and comment on it... but we didn't see it? What would they have said anyway?

Lorraine: Wow, Marty! You look just like a highschool friend of ours. (To George) Honey, c'mere! Look at Marty. Doesn't he look like that friend of yours, Kevin?

George: You mean Calvin? Oh yeah, he does! Shame we don't have any photos of him. Honestly, if he hadn't had disappeared all those years ago, I'd have sworn you'd been cheating on me!

Lorraine: Oh, honey! As if I'd cheat on you...

George: I know, dear. *kiss* Hmm. Is Biff done with that car yet?

What would YOUR parents say if you resembled a childhood friend they hadn't seen in 30 years?

It was 30 years ago for Marty's parents. I can barely remember what happened 30 days ago, I can't imagine remembering something from 30 years prior. Now given that Marty's parents encounter with "Calvin Klein" is what brought them together, I doubt they could remember what he looked like some 30 years later. I would say it's plausible that they would remember his name, but not his face.