Is this a mutant plant?

Yes, it is rubbish. You could spend a lot of time hoping and waiting for a miracle, but in my experience, such abnormal seedlings will either grow horrendously slow, or grow horrendously mutated. Honestly, bite the bullet, pull it out and plant some new seeds and try for a healthy normal seedling. It will cause you a lot less grief now, rather than to wait 10 weeks and still not have a normal plant. Not all seeds are perfect, unfortunately, you have found that one in a thousand that isn't so perfect, It doesn't happen often. but sometimes bad luck occurs. Hope this helps,... Organoman.