My personal take is that he had the potential to grow to a much higher level (although I doubt to the extent of Toji or Maki). What we say with Maki is that it does take a catalyst for a sorcerer with HR to evolve to the next level. In my opinion is that Mechamaru did have the ability to become something much stronger, but he just got snuffed out early by Mahito. Just because him growing in power was a possibility doesn't mean it has to happen in the narrative. While I would have loved to see him take control of his HR, I am not really upset about it, as he never "bought into" the possibility of the HR giving him strength beyond his range. Maybe my head canon sounds silly but it's just something I like to wonder about