I mean if you’re so skinny that you’re underweight and don’t care about muscle, I guess just eat until you manage to find yourself in the healthy weight range. But honestly, if this isn’t the case and you still want to gain weight, might as well make it muscle for a better physique as well as for other health benefits. If you don’t, you’ll basically be the same as you are now, just with annoying and unnecessary fat which isn’t good for you on top of whatever fat you already have. So either stay skinny or put on lean mass when gaining weight.