All I ask is that you don't wear your PJs to board the airplane and do you really need to take your own full size pillow for a 2.5 hr flight?
You don't have to look like you stepped out of the pages of GQ or your fav fashion mag but I have found that if your are dressed half-way descent you tend to get better service. Run of the mill chinos and a polo are fairly comfortable and are usually considered biz casual.
If you feel under-dressed then maybe your are. Spruce it up a bit to make yourself feel a little better. I am not one to get gussied up either and I do prefer comfort of style.
(The gate area isn't a play ground or romper room but that is another subject)