The polyp was flat and the doctor said he thought he got most of it out but he was not sure. are polyps in the transverse colon more likely to be cancerous.

Not more likely: Polyps can be flat (sessile) which are more likely missed by endoscopists and can be more dangerous because any existing dysplasia is closer to deeper layers. The location in the colon doesn't affect malignant potential greatly. Most important are presence of abnormal tissue and if present, depth of invasion. Many polyps are benign and only require more frequent colonoscopy.

No, however...: To the best of my knowledge polyps in the transverse colon have no greater risk of becoming cancerous than in other locations. Some varieties (adenomatous vs. Hyperplastic) are more likely to become cancerous. I'm sure your polyp will be checked for signs it was precancerous. Your doc will likely do another colonoscopy in about 3 years so if any was left it can be removed at that time.