Are Isekai mangas getting boring and lame?

Yes. But its to be expected since its one of the most popular genres in Japan and even if a good isekai is released, there is no guarantee that it will be noticed due to the extreme competition in the genre.

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Kinda unrelated but there's a good manga that isn;t isekai, but is set in a medieval-fantasy world called The Dragon, The Hero, and The Courier which kinda shows the daily life in that world. If you've read a lot of isekai, it really is a fresh breath of air in comparison.