Unless you are applying to certain fellowships and/or looking to transfer into a different PhD program, no one is going to care about your undergraduate grades once you're accepted into a PhD program.
That said, I strongly encourage you to stay disciplined. Graduate school and the rest of your academic career will be filled with distractions and reasons not to feel motivated to work--things like job market anxiety, anxiety about tests, falling out with advisors, etc., that are far more psychologically taxing than "senioritis"--and literally the only way you're going to succeed is forming a strong work habits that allow you to remain productive no matter what.
Sorry to sound bleak. I'm in a grad program and learned about self-discipline the hard way. Start building the habits now.