Yes, you can wear a black plain t-shirt and blue jeans to the JEE Mains exam , as long as they follow the general dress code guidelines provided by NTA:

• Simple, comfortable clothing is recommended for the exam.
  
  
• Avoid wearing any kind of accessories like jewelry, belts, or shoes with thick soles.
  
  
• Shoes : Closed-toe shoes are not allowed, so wear sandals or slippers.
  
  
• Clothing : Avoid clothes with large pockets, zippers, or embellishments.

Make sure to check the specific dress code instructions in the official JEE Mains information brochure to ensure you're fully prepared.

I hope this answer helps you. If you have more queries then feel free to share your questions with us we will be happy to assist you.

Thank you and wishing you all the best for your bright future.