People are worse at mental arithmetic than they were in the recent past, so it's not clear that they aren't "dumber" in the sense people meant at the time.
And did our thinking about the importance of being good at arithmetic change in response? I think so.
We also used to be much better remembering things, when we relied on oral histories, our memory skills have degraded quite a bit. And there's a quote from Socrates criticizing how writing is a crutch that degrades our skill (https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1... , the last bit). Over time, we've just moved to valuing other things more.
Well, with anything, practice is key. When I was in school, I was in a math competition where you had to do everything in your head. There was no scratch paper, you could not modify your answer once written, and erasing was obviously not allowed either. I wasn't the greatest at it, but I didn't suck at it either. That was decades ago, and I no longer do math in my head that way. What I used to do in seconds for a result now takes a couple of seconds to think about what needs to be done and then the time to come up with the result.
Students score lower on standardized tests in the 2020s than those in the 1990s. So your stance feels misguided. Although I don’t think Google and Calculators are the main culprits, I do think it’s due to larger technology/internet landscape.
I once worked for a guy who typed 7 + 4 into a calculator, after freezing for 1.5 secs trying to work it out in his head. It was in a "stressful" situation (not something extreme, we just were in a hurry), and I'm sure the guy could add those numbers in his head, generally... he owns his own business, after all. It took so much out of me to not move a face muscle.
Sounds like you haven't used it much. It starts small with you forgetting the arcane params to commonly used tools that you don't need to type anymore. Where it will stop nobody knows.