That’s patently false, and I’m not sure where you’re hearing that from.
Your grades are far more important than where you went to school. It’s not that your undergrad doesn’t matter at all - it does - but it typically only matters in the upward direction. In other words, while coming from a highly regarded department at UCLA might be an advantage, going to CSU-Northridge won’t be a disadvantage at all. I had some colleagues in graduate school who came from Cal State campuses. An acquaintance of mine actually went to CSUN for undergrad, went to a great PhD program in my field, completed her clinical internship at Yale and is currently doing a postdoc at UCSF (one of the top schools in my area).
If you like CSUN and you are happy and thriving there, don’t transfer because you think it’s going to make a difference. You can still get into an excellent graduate school with a degree from CSUN.