When using construction material for furniture the relieved corners effectively guarantee this. I would absolutely not use wood filler there, that will look much worse.
Aside, why were you taking clients before knowing how to make the thing?
#Should I use wood filler here?
half salmon
gleaming fiber
Appreciate it!
brittle viper
It would be helpful to set expectations with the customer when using construction grade lumber in a project. You have to remember that that kind of wood is for construction and the quality of it is not the best. As QuitePossible mentioned, construction grade lumber will have the edges relieved so you will not have nice square corners on it unless you run the lumber through your tables saw or planer to remove it. Pine is also not great when you put a stain on it. It will be blotchy unless you put a sanding sealer on it first. That will give you a more consistent look to the stain. Good luck!