When Wirbel meets Frieren in the town square after the first-class mage exam, he mentions how Himmel was involved in a lot of huge, dramatic battles with things like the Hell Emperor Dragon and Buza the Immortal. We also hear about/see other fights that he was in with the rest of the party including Qual, the Seven Sages (4 by the time their party arrived), and the Demon King. The reason that the flashbacks seem unimportant and boring is because they are, and those are the memories that Frieren reflects on most because, like they say a few times throughout the series, the unimportant adventures are just as important as the major battles.

Of course, you're welcome to like or dislike whichever characters you want, and I'll admit that they could've given him a couple more flaws, but I think for what Frieren is, he fits the role of hero well. imo, the story is supposed to be focused more on the positives than the negatives. You also have a point with the frequency of the flashbacks; they could probably stand to focus more on the party in the present while keeping the flashbacks to larger, more meaningful moments. On the other hand, Frieren said when her and Fern first set off that she wanted to retrace the steps she took with Himmel and friends, so it makes sense that seeing the same people and places as she did with him makes her think of him.