The phrase "lucky mouse" can refer to a number of different things, from a cartoon character to a successful piece of software. If you're in need of synonyms for this term, consider words like fortunate, serendipitous, auspicious, or charmed. Alternatively, you could use slang phrases like "in the money" or "riding high." If you're referring specifically to a mouse, you might try "smart rodent," "nimble runner," or "quick-witted critter." Whatever variation you choose, the implication is always the same: something or someone who has experienced unexpected success or good fortune.