> I had a lot of suggestions for improving the codebase
It's important to learn politics and humility.
As a new graduate (or junior in general) on your first job you should be extremely careful about making suggestions to 'improve' things. Heck at any point of your career you should be careful about doing that soon after joining unless you were brought in as an expert to do just that.
From this and the rest of your post, I would suggest that you try to develop a more diplomatic, political, and humble approach in your next job. This is an important skill.