Come now, I don't think it's bad
I agree. I would like to add a few points --
1. It's a central part of LOTR -- the cultures intricately designed by Tolkien, and creating languages (and cultures) was the reason he wrote the whole series to begin with.
2. She can sing just fine. Her voice is quite melodious. What's the problem, she doesn't sing modern songs in English from the 2020s? No thanks!
3. Related to #2, the song sounds
foreign, which is a GOOD thing because this is supposed to be Middle Earth, not America circa 2020. Her song in a foreign language pulls me INTO the movie rather than OUT of it. If she sang "Come on baby, one more time" it would violently rip me
out of LOTR. Instead, this song evokes the mysterious culture of Rohan, which the viewers know little about.
4. Parts like this are what made the book (and the movie) good. If it were all battle scenes, it would get tiring.
5. Suspension of disbelief was particularly easy, since these white boys and girls look very Celtic, and the ancient language used by Rohan sounds very much like Gaelic, at least to my untrained ear that doesn't know Gaelic.
The whole thing works. Well.
Very well.