eh I mean, morally speaking no. But then again morally speaking it's not ok to translate/repost any author's work without their permission, including mainstream popular magazines like Jump.
Though I'm not too familiar with pixiv, is it a free platform anyway? If so, I can't imagine an author getting too pissed with their works being shared so long as there's a link back to them/credit is given to them, which mangadex does. I mean, I would still cease translation if they actually contacted you about it and asked you to stop though. But if there is any paywall, that's typically why small scale/indie artists will frown upon it, because they're not making the same kinda cash a WSJ author would be making and therefore have more to lose from piracy.