Hi Umang,
I'm not familiar with Google Optimize, but it seems like the error you were receiving is a pretty common one. A quick web search of "MIME type ('text/html') is not executable, and strict MIME type checking is enabled" brought back several links from Stack Overflow and Get Hub with similar issues. From the threads I read, it looks like it may be a configuration issue with the script and the web server.
Have you tried deploying the Google library outside of DTM on a test site? If you still receive the error when deploying the code outside of DTM, we know that the error must be coming from the library itself.
Cheers,
Jantzen