Absolutely not. It’s your world; play how you find it most enjoyable!
Check the rules. Many competitive PvP servers or "Hardcore" survival servers consider Fullbright an unfair advantage (similar to X-Ray) because it bypasses a core survival mechanic (darkness). Always read the server's /rules before enabling it. Conclusion fullbright 1.12.2 resource pack
By setting the light levels to maximum across all textures, Fullbright effectively removes shadows and darkness from the game. Whether you are at the bottom of a ravine or in the heart of the End, your screen will look as though it is high noon on a clear day. Why 1.12.2? Absolutely not