To be honest, I don't think many people call him a bad singer. It's mostly just some toxic Exo-Ls because of fanwars and the need to assert dominance in the form of vocal lines 
In my opinion, his vocals are more overlooked than anything.
As one of the most popular members and the main vocalist, it seems hard to believe that Jungkook barely gets vocal praise, compared to the other members. But, if you think about it, it's true. Jimin and Taehyung are praised for their unique vocals and Jin is praised for his improvement and "silver voice".
People only compliment Jungkook's vocals in videos that specifically highlight them (eg. "Jungkook's best vocals").
In BTS' songs, he mostly sings the repetitive parts and the high notes are given to Jimin. His voice is often called boring, typical or emotionless (it's honestly not and I don't understand why people say this).
In Love Maze, Jungkook did a vocal run where he hit 8 notes in 0.8 seconds. That was probably the most difficult run in BTS' discography and he did it with such ease as well. You'd think the entire comment section would be talking about it but there were only like 5 comments at most that I saw.
I really agree with this:
But I do agree that he is underappreciated. He does the track guide, adlibs, brings balance to the song, harmonizes, and yet it does not get that much appreciation which is kind of sad
A lot of ARMYs only know how to cry about him getting the most lines without realising the amount of work that goes into being a main vocal. Jungkook (and possibly Jimin) carried BTS' vocal line in their rookie days but no one acknowledges it. He deserves more credit as a main vocalist because he is a damn good singer.