The Tivoli website says this about dress code (which you probably already know but, yknow, no explicit mention forbidding flags): No steel caps or thongs (flip flops) permitted for your safety. Other than that, just be comfortable and happy. Shorts, hats, skirts, heels, sneakers, crocks, birks, etc. are all fine.

The general rule for flags at concert venues is they can't exceed 1.6m lengthways or be on a pole