While tipping is at your discretion, I tip. The amount depends - one time I was delayed getting to the resort and they were "overly" waiting - 30+ minutes (pre Covid and you paid when you received the scooter) - that time I tipped $20, normally I tip $10.
I really don't tip for scooter rental. If you were using Disney's preferred scooter rental company the scooter would be waiting for you for pick up and the rental company wouldn't even be there. And it would be the same when you dropped it off. The fact that Disney forces other scooter vendors to do an in person drop off is not our fault. Those vendor companies know going in that they need to do an in person drop off and pick up; so, I believe they cover that cost in the scooter rental.
There are a few exceptions like if the scooter company did wait for you and you were delayed. Also, if you needed special instructions and taking up their time more then I would tip. Even so, I wouldn't go above $5.
When you are off Disney property most resorts allow the scooter vendors to drop off and pickup without you being there.
It honestly never has crossed my mind to tip the scooter delivery people. I guess if we were delayed I might, but I usually order it for the day after we arrive for early morning.
Sometimes I tip, sometimes I don't. Last time they picked up my scooter, they left so fast, I didn't really get a chance to tip. I'm more apt to tip at drop-off, because they coordinate time, explain things to me, etc.