The diagnosis for ADHD specifies multiple areas impaired. Just flunking class or getting fired won't do it. Unless we think maintaining basic friendships or relationships, suppressing destructive impulses, defeating procrastination, avoiding chronic disorganization (extreme clutter), staying away from addictive substances, etc., has also become significantly more sophisticated, I'd think the aspects that affect those types of activities would stand alone as an indicator.
There have been epithets/cliches for many, many years that come down to pathologically spaced out, lazy, foolish to the point of not being "normal"; and we've had "minimal brain damage" and other pre-ADHD executive function impairment diagnoses for a long time as well. It's not just a modern thing.
That said, it's also worth talking about which things aren't derived from changing standards, or at least recent ones. The perception of something like schizophrenia or suicide may differ between cultures, but both have been pathologized in the US and Europe since pre-industrial times. Less dramatically, any psychological condition which leaves a person largely housebound (e.g. extreme depression or some presentations of OCD) has been pathological since at least the end of the Victorian Era.
To the extent that we see a rise in suicide rates, debilitating depression and anxiety, and patterns like hikikomori and its international analogues, it really does seem like something has changed about behavior as well as society.