Yes and no. If you have a negative online presence, it can absolutely turn off agents and editors. If you have no presence, then you don't have any bad habits or negative impact issues they have to worry about. However, if you have an excellent online presence with a strong following, agents and editors notice that as well. Personally, I like to see an author that has established themselves in a positive and relative manner somehow online. It shows they are ready and willing to do the work.
Today a lot of successful authors not only have a great website, but they blog, Twitter, Instagram, Pinterest, or have a Facebook page. Do you have to do all of these things to have a strong web presence? Absolutely not. Like most things in life, pick the things you're good at and focus on them. If you like to chat in an almost texting style format, then Twitter might be for you. If you like to talk about all the things that interest you, post pictures, and such, then Facebook or Instagram might be for you. Blogging might be for you if you have something to share with others. The thing to remember about blogging is that if its all about you, people will lose interest. They want to read about something that interests them or that helps them in some way. Once you're published and famous, then they'll love to hear all about you, but save most of that content for that point in time. That said, there is nothing wrong with a newsletter that others can sign up for. In fact, every author should have one.
So should you wait until your published to begin all this? Not necessarily. Beginning these things now will allow people to get to know you a bit and start to build readers, which is what the publishing industry refers to as a platform. Even if you only gather a few people, at least you have begun. The best part about it is you will have started to meet people with similar interests and you'll have started to network. And as we all know, networking is key in the literary field!