As my cycling these days is mainly off road, if I discover fellow mtb'ers on the trail appearing in need of mechanical assistance I consider it trail etiquette to always slow / stop to offer a helping hand if needed. The majority of times I get a " Cheers pal, all good here ".
Sadly there's a section of the mtb community who don't do the rest of us any favours and just ride on by, but I suppose they must be the special ones who are better than other folk.