If the claim masks don't work didn't come from the same people who said isolating didn't work, I might take them seriously. I might not just see it as motivated reasoning or another lame effort to discredit or cast suspicion on authorities.
If it didn't come from the same conspiracists who see nefarious censorship everywhere or people who only see their personal rights being infringed over our social obligations to each other, I might take them seriously.
Or I could take them seriously if they understood the risk of confirmation bias by cherry picking preferable information when credible contradicting studies or meta-studies exist (like this one that states "The preponderance of evidence indicates that mask wearing reduces transmissibility per contact by reducing transmission of infected respiratory particles in both laboratory and clinical contexts." https://pubmed.ncbi.nlm.nih.gov/33431650/ ). Or the risk of socially promoting that one-sided certainty.
Or if they just understood the precautionary principle that if in the face of competing evidence, we could potentially all benefit from taking the more cautious approach at the risk of minor personal inconvenience, I might take them seriously. But I don't because they aren't serious people. And they aren't even slightly interested in applying the scientific method.
And they don't realise they are in the loud, unreasonable minority who have a megaphone they would never have had before the Internet, and they don't feel obliged to use that power responsibly. With time and education I hope they will dwindle in number, or at least shut up a bit.