Reusing fritter batter the next day is generally not recommended. The batter's leavening agents lose effectiveness, and the flavor can deteriorate.
Fritter batter typically contains ingredients like baking powder or baking soda to create a light and airy texture. These leavening agents react when mixed with liquid and begin to lose their potency over time. If you let the batter sit overnight, the leavening will be significantly less effective, resulting in dense, flat fritters. Additionally, the batter can absorb flavors from the refrigerator, and the ingredients may start to break down, affecting the overall taste and texture. While it might technically be safe to use if refrigerated properly, the quality will be noticeably diminished. For the best results, it's always best to use fritter batter fresh.
If you have leftover batter, consider cooking it all at once and then refrigerating the cooked fritters. You can reheat them in an oven or air fryer to restore some of their crispness, which is a better alternative than using day-old batter.