If depends, if you are thinking of a PhD, on one, hand, academia does tend to be elitist, and the “prestige” of the undergraduate institution is important. However, experience, having an idea what you want to do, and glowing LoRs are more important, and these are much more likely if you are in a university for a longer time. Having four years and three summers tp develop relationships with faculty in the field that interests is much better than trying to do it all over again after transferring.
An applicant from UCR with three years of lab experience, LoRs from faculty and post-docs who have known you for two or three years, the ability to have your own projects, which is much more likely if the PI knows you from your previous work in their lab, etc., is a top notch applicant.
Bottom lone, staying in UCR and taking full advantage of all your opportunities there is probably a better route to grad school than transferring to UCSD