I wouldn't want to be accused of cherry-picking the sample size, but this is how I normally do comparative studeies.
I'd figure most teams have 7-8 RP'ers at a time. Of course, they use many more over a year, and I could count every single RP'er who pitched to just one batter, but that does not seem fair.
I would take 8 RP'ers and multiply by 30 teams. That's 240 RP'ers.
I then go to fangraphs and move the IP requirement down or up, until I reach a sample size of 240. To me, this is more than fair, since it does not count the hundreds of pitchers not good enough to reach that threshold.
I might go year by year for every Barnes season rather than the full sample size of combined years, because some pitchers just started 2-3 years ago or retired 2-3 years ago and would have skewed numbers due to timeline disparities.
So, let's take 2018- Barnes' middle fWAR score from 2017-2019. I had to go to 30IP+ to get a sample size near 240 (250, in this case)
If I divide this sample into 6 categories, it would be 40 per category.
Kimbrel placed 27th and would be in the top tier of 40.
Barnes placed 38th and would be in the top tier out of 6 tiers- VERY far from average (tier 3+4) and extremely far from below average (tier 4).
I'll go ahead and do other seasons:
2017 (30+ IP creates a sample size of 235)
Kimbrel was #2 and clearly top tier.
Barnes was 57thy, which places him in the 2nd highest of 6 categories. (If I went to 8 categories of 30 pitchers, each, he'd still be in the second highest of 8.)
2019: (249 RP'ers with 30+ IP)
Workman was 5th (top tier)
Barnes was 23rd (clearly top tier of 6, 8 or even 10 categories broken down into equal parts, and remember, I'm not counting all the scrubs with 1-29 IP.)
Let's go even farther back, when he was not as good:
2015: (only 214 RP'ers with 30+ IP)
Uehara 21st (top tier with 7 tiers of 30 pitchers)
Tazawa 37th (second to top tier out of 7.)
Barnes was 194th (bottom tier)
2016 (223 sample size)
Kimbrel was 31st (top tier with 6 tiers of 35 each)
Barnes 107th which places him in the 3rd tier out of 6: 2 higher- 3 lower, but just barely a smidge above average.
Now, I'll do what I said can be misleading. I'll go from 2015-2020, where a sample size of 130IP+ is needed to reach 245 pitchers.
Barnes places 51st out of 245.
That puts him in category 2 out of 6. Category 2 out of 8 and category 3 out of 10.
If you want to hold 2015 and 2016 against him, or 2020, fine.
The guy has been above average when you compare him to all RP'ers of his time. That's how everybody I know views "average" or better or worse: 3 categories, 5, 6, 8, 10, whatever.
He's been better than the vast majority of other RP'ers, but thyat does not mean he's better than most everyday players or SP'ers.