Explaining Poorly Chosen Major and Average Grades I am a Mechanical Engineering major applying for Ph.D. programs in Physics (in the US) who was always interested in Theoretical Physics but chose Mechanical Engineering due to a poor career choice at the time of the selection of undergrad major. Being interested in only theoretical physics, I didn't care much about perfecting the engineering subjects and this is reflected in my grades. I have very good scores in Physics courses (except for an applied physics course). I am writing my SoP and have gotten advice from a lot of people that I should explain my choice of a different major despite my interest in physics as well as my average grades in engineering subjects. But I am not sure whether plainly telling them that I got viciously lured into engineering by being told "undergraduate physics education in our country is not good and one can easily switch to physics after the undergraduate program if one has done undergraduate studies in engineering" would help. Also, whether plainly telling the truth that I didn't find engineering interesting and thus, my grades are average would help. It might (wrongly) show that I would not study uninteresting things that are required for some research project. 

I have a fair amount of content to write about my interest in Physics, for example, I am pursuing a minor in Physics, I have done a good number of interesting research projects, I have attended some schools on theoretical physics, have taken a graduate course in GR, I have well-identified research interests, etc. etc. Is it required (or helpful) to write about why I chose a different major and why my grades are average in the engineering subjects? If yes then what should be the tone and approach?
I do think you should explain your mediocre grades in your major in your application.  Give the honest truth that you feel it was a poor choice of major and you lacked interest in the subject.  It's not perfect, but it's better than no explanation.

The more you can portray it as a story of growing and maturing over time, the better.  Focus on the positive: lessons learned, developing more discipline, finding your passion in your physics classes, etc.

I don't think you should say you were "viciously lured" into engineering.  It doesn't really add anything to the story, and also it comes across as blame-shifting.