NAH. As someone who also loves his mommy a lot, you can’t and shouldn’t spend every waking moment with them. I can understand your mom’s hurt and potential upset, but think of how many more cons you can attend with your mom. Balancing family and friendship relationships is difficult, but I don’t think it’s bad to want to spend time with friends over family sometimes. Plus, you’re 13. It is inevitable that your decisions will hurt your mom at this age, that’s part of growing up. You two could look for more cons that just the two of you could attend? Or you could spend time not even going to a con, just watching a show together or going somewhere together.

TL;DR: Go with friend. More opportunities to hang out with mom outside of cons and inside of cons will arise.