Yes, mattress stitch, no more than one stitch in from the edge.
How did you work your decreases? If you did the SSK or K2tog on the very edge its harder to get a neat seam, and you need to take it very carefully. To avoid a bumpy irregular edge, which is hard to seam neatly, decreases are better worked one stitch in, ie K1, SSK, and for the end of the row - work to last 3sts, K2tog, K1. There will be a slanting edge as in the pic of my baby hoodie -
Top down knitting is great, but you will find a much greater variety of gorgeous styles are possible if you do your knits in pieces. With baby knits the seams aren't very long, but I know a lot of people find them tedious. I think they're worth it for the finished product.