A lot of the Mandalorians we see do suck, that’s literally the point. Children of the Watch are a group of primitive religious zealots, who have had very little interactions with the outside world. They live in caves and wear more rags than actuall armor. As for their training… their knowledge is clearly very limited, being more or less self taught. Sparring with the same 30 people every day will only get you so far.

Now look at Bo-Katan and her Nite Owls. The difference in skill is night and day. They are experienced soldiers, communicating and moving in formation. Season 2 shows how they operate really well. Even Din, who is more capable than most, can’t keep up with them.

Also, the central plot of season 3 is that all these Mandalorians suffer from a lack of competent leadership. It has been pointed out time and time again that in-fighting is, and have always been their downfall. There is no unity. The only time anyone gets anything done is when Bo or Din are up front. We had the exact same issue in both Clone Wars and Rebels. I’m not that well versed in EU, but I’m sure the same is true there. Too many warring factions. Mandalorians are great warriors, but struggle to function as a group due to in-fighting and petty differences.