If you were planning on using Mathews on your bench, take this trade and don't look back. Bernard's upside is just so much higher than Mathews IMO and Ballard isn't exactly bad to have on your bench.
If you were planning on starting Mathews, I'd be a bit more hesitant but I'd still do it. I think Bernard will win the majority of the carries fairly early on, but it's obviously a risk to go for a guy that is going to be splitting time with another RB as opposed to a full-time starter. Still, we've seen what Mathews can do, and it isn't pretty. And if your team was autopicked, I'm guessing that it isn't very good and full of "safe" picks. So, I wouldn't try to be risk averse here and would want to go for someone with more upside. I'd need to see your roster to give a full evaluation, but those are my thoughts at the moment.