Is it normal to experience dissociation of your environment and the people around you many times a day?

No: No, it's not normal to dissociate many times a day -- but many different conditions can cause symptoms that seem like dissociation. Medications, other substances, anxiety, ptsd, seizures, etc. Your information indicates "astrocytic brain tumors" with past brain surgery, and being on zoloft (sertraline). Please tell your doctors what you're experiencing.