Colonoscopy/Biopsy report: the lesion (0.5cm) is lymphangioma of the rectum, Solitary rectal ulcer syndrome, and mucosal prolapse syndrome. Is it bad?

No, nothing bad! It is likely that your rectal mucosal prolapse caused the solitary rectal ulcerr syndrome and needs no treatment unless there is significant bleeding or discomfort which you did not mention having. The incidentially found 5mm lymphangioma is totally benign and should be of no concern. That lesion is exceedingly rare with very few cases reported and never a report of malignancy in them.
Not usually. Think of it as a solitary hemorrhoid. In most cases it is benign and requires non surgical interventions of diet modification and stool softeners, sitz bath etc Pl see attached for additional details.