Why a capital ship would have tightened St Nazaire's guard

Would a German capital ship at St Nazaire have made the British raid less expected rather than more difficult?

No. A major warship attracts attack and therefore also attracts extra attention and defensive preparation.

A capital ship is a conspicuous tactical asset that an enemy will repeatedly try to destroy or disable. Its presence encourages defenders to concentrate attention around the anchorage and repair facilities. Once the major ships moved away, German planners could expect British attention to shift with them. Operation Chariot exploited the gap between the dock's strategic importance and its reduced day-to-day defensive focus.