As martianmister pointed out, viewership ratings for the Pokemon anime actually don't go up during the league. The lowest rated episode of XYZ and BW Season 2 both happened during the league.

What Dogasu said about streaming services is not wrong, but I believe we should still get some insight on how streaming services are faring in Japan. Which is why I shall (shamelessly) copy over a post on ResetEra detailing how the Pokemon anime is faring on channels not named TVTokyo:

One thing I've noticed is that not catching a show as it airs and contributing to live viewership numbers, isn't inherently nearly as bad anymore. In fact, it's how most kids are watching anime that would normally air during the day, but often instead are subjected to late night only airings, due to the lack of Day Time schedules available to them.

Dr. Stone, Kimetsu no Yaiba: Demon Slayer, JoJo, and a number of others, over the years, have been subjected to lay night schedules, despite the primary audience for the shows are intended for Boys 8+. However, what seems to have helped a LOT is that these shows often have high DVR numbers/points, meaning a lot of people across Japan are watching these, either through the Terrestrial broadcasts, or the Satellite broadcasts, just not live.

My friend had commented that stuff like JoJo was fairly popular with Kids in school (Upper Elemntary, Middle School range), despite the late night nature of the airings. Another thing is based off of research data they conducted, based off of surveys they did with elementary school kids about what dream job they'd want, seemingly Demon Slayer was incredibly high on the list with Elementary School kids.

So kids are still watching these shows, regardless if it's live or not. How they're consuming them has changed a lot, though, compared to years ago. It helps that DVR is a common function of most tuning devices compared to back when a lot of these shows were first airing. You could record it on tape back in the day, of course, but those weren't always 100% reliable, much like with anything.

Given the points Pokemon receives each week is about 36 to 50 (out of a 100) range, it does seem a lot of people are indeed watching Pokemon, but through DVR recordings. And this is incredibly important and likely something companies look at more than just live numbers anymore.

Now for some additional input for further context from myself:

DVR stands for Digital Video Recorder, which allows for people to record shows on TV. What is of note here is that we get weekly data on what the most popular shows on DVR.

From what I understand, getting a 100 on DVR basically means that show was the most popular on DVR. If Pokemon gets 36 to 50, that means it is about 36% to 50% as popular as the most watched show on DVR.

Finally, you can get data for the top 10 watched shows of each genre in the following two links, which goes all the way back to late 2014:

In conclusion? There is no denying that the current timeslot for the Pokemon anime is just plain stupid, no arguing against that. What is important though, is that people do still watch the Pokemon anime.