Rule of thumb: 1 astronaut-year spacewalk = 1 dead astronaut
Just be glad the telescope's not in LEO. You get gravitation pulling more natural particles in towards you and space debris too.
IIRC, the impact hazard from space debris starts getting noticeably higher than natural particles at around 0.1mm size (estimated size you can protect an astronaut against), but common knowledge back when I was in the lab was that you'd get about 1 astronaut-killer impact per astronaut-year of space-walk, so with JWST being 25m2, I'm not surprised it's got a few noticeable craters. (as well as hundreds of less-noticeable ones).
As for protecting the mirror.... that's a tough one. If it were my mirror and you gave me the choice of 'protecting' it with 1mm of Aluminium/plastic/whatever or exposing it directly to 0.1-3 mm particles at 66km/s (Orionid meteor shower), then I'd almost certainly choose to not have the inadequate 'shield', and just hold the mirror edge-on to the stream, so as many of them miss as possible.
Each impact on any shield is going to make a shower of ejecta (inside and outside, if the shield isn't thick enough), as the particle blows itself and the target to plasma and makes a pretty crater. Impact ejecta like that can cause a massive amount of damage to surrounding surfaces, far more than the initial impactor would.
To properly protect it, you either need a few tens of km of atmosphere or multiple layers of stuff to break it up, spread it out, break up and slow down the high-speed ejecta, spread that out, etc. How big was that telescope again?
(Fun viewing: https://hvit.jsc.nasa.gov/impact-videos/, but remember children, this is lab simulation of some slow orbital collisions, certainly not head-on, and let alone the impact speed of any natural meteor streams.)