Do bad endings ruin everything

I believe it also depends on if the media itself is also bad or not

I hate to bandwagon, but as mentioned above, "it depends." I can think of multiple anime/manga examples personally.

I DO think bad endings that are bonkers can help certain pieces of mediocre media in rare cases. For example, I think of Master of Martial Hearts and School Days having endings so PUTRID they became MEMORABLE, if not outright memes (like no one would remember these shows if they didn't just basically go absolutely HAM in the end).
Dangaronpa V3 ending anyone? I loved the game but personally didn't like this one, but I can't hate it because it still is in the SPIRIT of the series. It definitely did an "ending" differently than the anime, which was interesting. 

It is well known that a number of 90s/00s anime suffered mediocre endings due to too long hiatuses in the manga and/or the budget/contract running out before the manga actually finished.

The Digimon Season 2 epilogue is notorious, though I'm pretty sure it's been fan discontinued at this point.
I still don't know how I feel about the Death Note ending. I think I do feel better about it than some people. Same with the also notorious Princess Tutu ending...I'm very torn on THAT one. It's tough because I see both sides too easily.

And how about a whole ending SEASON feeling "odd?" I feel that way about Sonic X's "3rd season," which feels so different from the original 2 seasons that it might as well be its own thing (parts of it feel like dang GUNDAM IMO). But that's likely because the third season was actually commissioned by a company rather than just planned outright, and is not based on any of the games. In a similar vein (though I've not finished it yet), I had heard the final season of Yu Gi Oh GX gets bizarrely dark and odd too.

To go back to the original question, DO bad endings (or ending seasons) ruin shows? I still think "it depends."

the funniest part about the Voltron story is just how much effort they put in to convince us they were doing something good for LGBT folk

they actually stated in an interview they learnt about the bury your gays trope, tried to avoid it, only to bury a different gay in the process

here's the interview btw. it came out a week or so before the final season aired. if you know what they did to their LGBT rep in full this interview is hilarious
www.vulture.com/2018/08/voltron-legendary-defender-shiro-gay.html

GX season 3 was already dark and season 4 actually felt somewhat lighter to me but it also felt odd, some people didn't like the return of Nightshoud or the asspull explaining Zane's heart attack(it was actually due to the Underworld deck and not all the shocks he got during his underground duels which many people felt was a lame explanation)the Jaden vs Yugi duel was neat but the conclusion was kinda botched.

i also wasn't thrilled about the ending to DBZ and i've seen a number of fans feel the same way.

Yeah, I liked the ambiguity of the ending personally. It captured the "floaty" and "spontaneous" feeling of the show well.

It also did the "answer basically no questions and just VIBE" thing WAY better than Lemony Snicket did in his A Series of Unfortunate Events series.
(For the record, I really DO NOT LIKE the ASoUE ending)

I sure am glad they left it like that and never made any series which actively tarnish the original in hindsight...