That is normal for a stainless steel single wall exhaust on any four stroke made in the last few years. These engines are designed to run lean to pass all emission standards. The warmest the exhaust is going to get is for the first little bit of start up when the engine is running leanest because more air is supplied to the engine for cold starting. I had a wr450f and a TTR250 that would glow in the night time like that and my ttr had over 18,000 miles when I sold it.