Fed nonfat yogurt to baby for a week without noticing. She's 8.5m. Are there going to be long lasting effects to brain due to no fat?

No worries. An 8 month old will be getting fat from other sources, such as breast milk, formula, etc... As long as there is a normal, balanced diet that includes fat, protein, and carbohydrates suitable for an infant, there is no need to worry.