Is moss good or bad?

If it's on the soil then that's not a good sign, do a soil test to see if the soil is acid as I think this encourages moss......and are your beds in deep shade, cos that won't help either.
Compacted soil gets a lot of moss as well?

I thought lichens were supposed to be a sign of clean air not moss 

No, there's no drainage problems on the beds, and neither are they in shade. It's actually raised beds filled with sand-based imported topsoil just a year ago, and the drainage is great. It's regularly watered because it's a veggie bed, so certainly on a dry day it'd be damper than other beds that don't get watered, so maybe that would be an attraction for them. And whilst the bed itself is not in shade, at moss level they would be in the shade of my veggies. I wouldn't have thought there'd be any compaction either, as the beds are never walked on. I believe moss and lichen are both sensitive to pollution, and I think the two generally tend to do well in a similar environment.

But I was looking at the stuff today, around the base of my mangetout plants, which are still going great guns and still cropping even now. It is definitely suppressing the annual weed seeds because it's covering the bare soil, so if it's not taking away nutrition from the mangetout then it must be doing more good than harm.