Those graves in Mariupol can be dug up and I don't doubt that the numbers will likely be thousands higher, but remember the Ukraine conflict is entering it's third year, the Israel-Gaza conflict is about 4 months old. If Russia was targeting civilians they're doing a rather poor job of it, where as Israel could do a much better job of avoiding civilian deaths yet they just don't give a ****. Yet we support Israel while hysterical people call Putin the next Adolf Hitler. The rest of the world sees us doing this by the way.
Lots never saw a grave - amongst other things there were dozens of apartment buildings hit with 100s sheltering in the basement, etc. which were simply bulldozed and the rubble removed along with any bodies.
Sentiment is turning against Israel, they certainly don't enjoy broad support for example https://www.bbc.co.uk/news/world-us-canada-68177357 but ultimately it is a different situation, even if Israel were trying their best, which they certainly aren't, there are going to be a lot of unavoidable casualties in that situation and Hamas and so on have made a situation where Israel doesn't have the luxury of just letting it go.