You should try putting a fan in your grow with a timer, so that the plant gets some movement to stimulate a stronger stem. Personally I train my plants every day (wiggle them around and check out the tops).
A fan is a good idea, keep it on 24/7 It's probably thin because air movement is minimal due to the walls of the pot. I would transplant into a larger pot and fill it to the brim.
its normal that the stem is soft at first, its gonna get woody pretty soon. if you only got one plant you dont need to get a fan, you can just lightly wiggle and bend it with your fingers. if it falls over, put a mini stake such as a paper clip with a loop bent around the stem in there. your plant looks healthy
it looks ok, a little small for its age, I have a hunch that your pot is a bit big and your not getting a fast wet/dry cycle and the roots are getting a little suffocated but other than that it looks ok. the stem looks pretty normal for this stage but it is very important to have a mild breeze from a fan on it 24 hours every day to keep the respiration rate up. still air will slow its respiratory system, essentially suffocating it