Should a user be notified about "last modified" time of bitcoin.conf while the bitcoin core wallet is loading? I have mentioned details here: https://github.com/bitcoin-core/gui/issues/76
Also what would be the best approach to solve this problem using c++ which works on linux, windows and mac without any issues?
Few solutions are mentioned here: https://stackoverflow.com/questions/40504281/c-how-to-check-the-last-modified-time-of-a-file
No.
If your machine is compromised you can't trust last modified timestamps.
One of the obstacles to Bitcoin adoption is complexity. Developers should keep the user interface as simple and clear as possible. 99.9% of users wouldn't understand the information or how to use it.
A better approach to detecting malicious changes to a configuration file might be to encrypt the configuration file with the wallet password or to store a checksum of the config file in the encrypted wallet file and only alert the user when configuration changes are detected that were not made via the wallet software itself.  But any message should carefully and clearly explain what the situation is without confusing new inexperienced users.
I'm not sure how useful this is since if your wallet config has been maliciously altered the whole computer is untrustworthy.