This is a sentence that I saw in the subtitles. What is the reason that ちゃった is used here. I know that it used for regret/accidents/completed action, but it seems like something positive happened here.
You're right ちゃう can be used to express an accident or something that is done completely. And usually it's under the context of a negative light, but it doesn't necessarily have to be. It can be used in a positive light too.
いっぱいいいねもらっちゃった can be seen as something like "It (the picture I took) received a ton of likes! (I wasn't expecting that! / Who would have thought this would happen!?)"
Receiving a lot of likes could still be seen as an accident, even though nothing negative happened, because you weren't expecting it to actually happen. Which is indicated by the ちゃう