you can have a meteor shower w/o celeste appearing, and celeste can appear w/o a meteor shower occurring! they may seem like they should always come together, but they can come separately as well
i haven't had a meteor shower ever since the first week of me playing. i usually don't check in during the 8pm time since shops are closed soon but even after tt i haven't had much luck
Any time I've seen or been to a meteor shower (announced by Isabelle in the morning), Celeste has been there. Showers announced by Isabelle equal a crazy amount. When you're walking around town during the day, and villagers mention shooting stars, Celeste has been there. Showers announced by villagers equal a good amount. If Celeste is there with no mention from either party, small showers.
I do think having KK and Redd made Celeste's visit algorithmically impossible.
The two are not required to go hand-in-hand with each others. And it's possible to have some shooting stars without a shower happening at all (though it's extremely rare). You're pretty much guaranteed a visit from Celeste at least once a month for the Zodiac recipes, but depending on the time of the year, there's a low chance of you having a small or large meteor shower.
The first time Celeste ever visits you, you're seemingly guaranteed some shooting stars, but after that, it's completely random. According ot the guide book, there are some days of the year where it's impossible to have any shooting stars at all (Cherry Blossom season severely reduces your chances, for instance).
If you have a shower and it's not Celeste's first visit, consider yourself very lucky.