Is my fatty banana viable?

Could be genetics, could be a balance of nutrient issue. Have to consult a leaf chart. one is linked in my profile comment section. picture is hosted on this website too. for example, too high of P or K could cause thinner blades in this case i'd give it a moment before making and judgement. It's definitely not a big deal. No worries. If you see other symptoms appearing, then you can worry. save a copy of that leaf chart i mention... over time you'll get familiar with it. start learning what you put into your substrate.. how much of each nutrient? etc.. Your pH says 6. This is too low for a soil grow. Get it up near 6.8-ish. This could be the cause too - via impact on availability of nutrients. Definitely get that in order and allow some time to see what impacts it causes.