Depends on the story, I don't have a discerning opinion for it.
Bad ending for me it's when you know that there's unanswered questions in the plot the author could've shown the development if a few sentences, but avoided it because whatever. Like, I really wanted to know what happened to that trio of secondary characters that were prominent in the arc and were forgotten. Or simply axed ones, where "our adventure continues!" shown as end in the manga, but in reality author has 10 arcs written in the light novel that has a mildly satisfying ending at the end of it.