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Depends on his position relative to yours. If he's the reviewer, only leaving thumbs down on a PR means that he's not doing his job properly. If he isn't the reviewer, you can just ignore him. When he tells you that your decisions don't make sense - if it's something that you're discussing, consider that he might be right and get in more depth on why your decisions might not make sense, especially if this person is more senior. If it's just a peer who's throwing around negative one-liners - just ignore him. If it's someone that you're supposed to learn from / be managed by - then they suck at their job.

Either way, don't quit because of this. It's far from what I would label "toxic" and it's likely that you'll meet people like this in most places so you're better off learning how to handle them early on.