No, its legitimate, and can be significant, but is it `mental illness`? A normal psychological response to stress is unpleasant, and maybe some people need help dealing with it, but its not not illness per se.that's very simplistic if not entirely wrong. mental illness is often mischaracterised in the sense that being sad = depression. however prolonged periods of low-mood will lead to depression, and high-anxiety is linked to almost every severe psychological disorder. and surely you're not contending that depression that "relates to external events" (does anything in the world not relate to some kind of "external event"??) isn't legitimate?
An anxiety disorder, and experiencing anxiety due to lockdown are not the same thing. Although, if you have an anxiety disorder, lockdown could well make it worse, but so could a global pandemic.
People are stating that lockdown causes people to be depressed and anxious, and equating that to some sort of mental health epidemic, but it isn't, its just a population that's miserable, but not necessarily unwell mentally. Being sad because your dog died is normal, not depression.