Mages aren't always the strongest characters that you could play. Do you think Sneak would be worth it or do you think it would be a waste of time?

if you get silent casting and use spells like fury and calm to your advantage then yes, it would be useful.

With illusion yes

I find that sneaking can be very effective with Destruction mage. You can play like a sneak archer. You can sneak to get in position where you can take out a couple of enemies with surprise attack. Dump all your magicka on the attack to kill them. Your attack range can be longer than an archer if you use an AoE spell like fireball. If they are all dead, you are done. If they are not all dead. You can retreat a bit and hide until your magicka regenerates to full to repeat.

You don't need perks in sneaking if you play this style, but your sneak will level.

This archetype is often referred to as a Nightblade.