No, it's not. Use whatever works to keep you tobacco free. Many of us started at 36mg before going to a lower level.
vaping is not nearly as efficient in getting nicotine into your system as smoking is.
For many, when you transition to a variable watt APV that allows you to adjust the warmth of the vapor, they find it easier to use a lower nic level e-liquid. But there is no issue with using 24mg eliquid if it is not giving you any unwanted side effects. Your body will tell you if you are getting too much nicotine way before it would be a serious issue.
And just in case someone comes along and gives you some inaccurate information, nicotine provides 90%+ of the throat hit in e-liquid. PG allows more of the throat hit to come through and VG mutes it to a small degree. PG by itself provides no throat hit.