I suppose there are different levels of willingness in the "willing suspension of disbelief."
The scenario you describe seems superficially plausible. I.e., many of us have had the experience of failing to recognize someone (even a close friend or relative) due to a novel context or some minor physical difference (new hairstyle, clothing, etc.). So on that basis, we imagine that the Clark/Superman ruse could be effective. But while this sort of "hiding in plain sight" is often able to fool on the first glance, it typically fails on the second or third. Thus, the notion that a main character (Perry, Lana, et. al.) could have
extended interactions with both Supes and CK and
not see through the minimalist disguise strikes many as, well, silly.
More generally: I'd claim that we, as audience, are usually fine with suspending
our own disbelief — because that's a typical requirement of engaging with fiction (especially fantasy fiction). But it's harder, I think, to suspend disbelief
on behalf of in-universe characters. So when these characters display less than expected levels of common sense or observational skill, the implausibility (of, e.g., the glasses disguise) becomes more conspicuous.