MIL & husband believe my milk isn’t nutritional enough for baby?

I BF except for 1 bottle of formula at night. My MIL came with me to bubs 4mo pediatric appt. He was at 30th % for weight. He was higher before but I don’t remember what it was. We have been struggling with sleep, and I was venting to her. She responded with “I have been thinking your breast milk may not have enough nutrients to carry him for long”. I don’t have the best diet, I pretty much eat whatever I can get my hands on (lots of bread, cliff bars, cheese). I’m not the greatest cook either. I told my husband this and he agreed that I need to eat more protein and he’ll start meal prepping for me. Maybe I should know this but does my diet greatly affect my milk? I still take my prenatals.