The M60 bus picks up at New York LaGuardia Airport (LGA) Terminal D, then C, then B. I know in the evenings it's incredibly crowded. How is it midday?
I have taken the M60 to/from LGA, as well as the Q33* and Q70. Mid-day is less crowded generally, but there are no guarantees for anything.
Also, very important, major construction just started at LGA.
As for the M60, since it is so popular, it sees a lot of frequency. If it's crowds you want to avoid and can spare a bit of time, you will often see one bus arrive <1 min after the previous. Since most people get on the first bus, the next one is noticeably less crowded. The M60 equipment also has luggage racks.
*Yes, I know the Q33 no longer serves LGA.
I'm not a local and have never taken this bus, but in general reviews confirm your view that in the mornings and evenings this bus is at best a nightmare.
It's not easy to filter those reviews for time, but I found one that was around noon (slightly earlier) and it was very positive:
I had to write this to try to swing the balance of these posts in favor of using this service. I landed at LGA and was at Rockefeller Center less than an hour later. I landed at 10:30 am on a Friday, so there was minimal traffic, and the Q train I caught was not crowded [...]