I wouldn't worry until at least 18 months unless there are other issues (peeing, pooping, growing, energy level, etc.)
The more you stress about it, the less likely it is to go as you want it too. I know it is really hard not to worry, and then hard to hide that anxiety from your child.
I really think what's best is to sit down together for regular meals, offer plenty of healthy choices (I would put puffs and mac-n-cheese in the occasional category, myself) and eat with your child. Do not try to talk your child into eating, or push food on her. At 13.5 months, she really is still just experimenting with solids. My dd is a HUGE eater, but at that age she wasn't eating much. I didn't really do purees, I just gave her soft finger foods and let her go to town. Steamed fruits and vegetables, raw fruits and vegetables, steamed rice, oatmeal, beans, scrambled eggs, etc. I did spoon feed certain messy things, like soup and yogurt, but she always had finger foods available to her, and I never pushed anything on her. When I gave her meat, I just minced it finely and put a little pile of it in front of her. Fish is easier for them to chew.
Model good eating habits, and share your enjoyment of food with her. I don't think it's great to have food out all the time. My kids eat a lot, but if they have food available constantly, it just goes to waste -- they get bored or sick of it, they play with it, or whatever. If we have a schedule and sit down to 3 meals and 2 snacks a day (or whatever) they are much more likely to eat the food and then go about their business. If they are hungry and ask for food, then I give it to them, but I don't just leave it out all the time. That said, if you find that leaving a snack plate on the coffee table or wherever she can reach it works for your family, then go for it.