Toyota has a lot more debt and a significantly higher debt/equity ratio than Tesla has, a merger shouldn't have a material impact on credit ratings.

Also, I feel quite confident in stating that the short-term market cap of the two companies merged would be significantly higher than the two companies separate. Toyota has a P/E of 10 (!). A merger would make it "cool", giving it a better ratio. Not Tesla's ~50 (based on quadrupling the last quarter earnings; TTM earnings are negative), but something higher than 10. And since Toyota has significant earnings, increasing the multiple of a big number....

Long term I'm not so sure it's a good strategy but a superficially the synergy looks awesome: combine Tesla's innovation and hype with Toyota's ability to deliver high quality cars.