Elon Musk is certainly innovative, but he's certainly not a saver. He seems to like to take resources to grow as quickly as possible instead of growing at a controlled, steady pace like larger companies do. Basically, he's more "go big or get more funding" than "slow and steady wins the race".
Jobs was pretty good at design and hyping up modest improvements. Elon just wants to wow everyone.
Apple is known for its operations. Tim Cooke was head of ops before he was CEO, imagine Tesla produced the same cars as today, but when it wanted to ship its inventory to Europe it didn't tank an entire quarter of sales because of fuck ups? Or imagine if Tesla had a competent scale-up of manufacturing their Model 3. Those are the things that Apple could help a lot with.
Apple's limitless access to capital would be one. Tesla has really had dial down many of their ambitions because of how expensive it is for them to raise capital.
That said, if Elon Musk had taken over apple from within (like steve jobs did via next) apple might have gotten more interesting.