The tab group UI looks very poor in Brave compared to Chrome (possibly due to the smaller height of the tab bar) - see below.
Granted this is just my opinion, but I think the following tweaks should be made in order for this to look better.
- More space between tab label and the first tab
- Reduce height of the colour outline
- Add some space between the outline and the tab
- Rounding the label and adding padding to either side will make this look nicer