I think it is a boxelder bug, are the bad for the garden?

Not bad for the garden unless you have a lot of box elders :-) And even then, more of an annoyance than a real pest.

They will damage fruit. But otherwise, mainly a nuisance pest.

Yes a nuisance, don't really do any plant damage but really gross when they descend on the sunny side of the house en masse seeking winter shelter.

Here they are on the outside of the bathroom window one year -- this was nothing compared to the heaps of them covering the siding. Blech.

That is where they were, sunny side of house!