Whether Charizard is overrated or not depends on how much you like Charizard, I guess. That's down to aesthetic taste. I do think it is getting a tad overused at this point, but I don't think it is especially overrated.
I find calling Kanto "overrated" to be somewhat ironic though. I am the last person who will hold up classic titles as the pinnacle of the genre - in fact, I loathe doing this, and it's something of a pet peeve of mine when people hold up old games as things that cannot be surpassed - but I will concede that video game history is important, and without Kanto, the trends it set, and its success, there would be no Pokemon as it is today in all probability. Calling Kanto overrated is basically calling the Pokemon franchise itself overrated, because the games owe their success solely to Gen I's success: if they didn't do what they did, we would not have had sequels. Or the franchise would have been relegated to a low-tier, low priority Nintendo franchise, like Kid Icarus or, god forbid, Ice Climbers. The best it would have been able to hope for would have been Metroid levels of popularity; i.e. a small cult following that gets very loud on certain areas of the internet at times.
Bringing Kanto as a region back repeatedly is not really a negative either because each time they have attempted to modernise the gameplay...which, if you've played a Gen I game, you'll know it REALLY needed. There's a pretty big jump between Fire Red and Red, and an even bigger jump between Let's Go! Pikachu and Fire Red. The older the game, the more it needs to be brought up to modern day standards, and when you're setting up new gameplay trends and standards - as they did with Let's Go - it makes perfect sense to start at the beginning, so newer fans get to experience the franchise in its entirety, and older fans get that taste of nostalgia that basically drives the entire gaming industry forward, for better or worse. Let's Go! is a spinoff series, whether the devs care to admit that or not, and once you take that out of the equation, Kanto has had ONE remake, the same as Gens 2 and 3. On one Nintendo console. That's hardly grounds to call it overused, which is what people usually mean when they say something is overrated. Yes, you can re-tread Kanto in the Gen II titles, but that is postgame, and it's one of the reasons Gen II is so highly praised.
As for Kanto Pokemon getting more evolutions, pre-evolutions, etc etc - well, part of that is modernising them and making them relevant in the current generation; there are a lot of Pokemon now after all. The other part is probably because of popularity polls in Japan and maybe in the West as well. I expect there is some game balancing in there, and...well, Kanto Pokemon all have very simple designs. They're probably much easier to adapt than later generations of Pokemon, and if Game Freak have proven anything, it's that they're a ridiculously conservative company that likes to cling to their past successes in the hopes that they can stay relevant in an industry that has completely changed whilst they have remained exactly the same. I'm not saying it hasn't worked - it clearly has - but...well, can you blame them? If it sells, it sells. Later generations just didn't perform as well in terms of sales - which is all they care about - so of course they're going to try and keep Kanto as relevant as possible, because Kanto makes money.
So...no, I wouldn't say Kanto is particularly overrated.
