Episode tags I don't think do much, but series tags do. If you search "knights" with the Tapas search function, my comic comes up, because "knights" is in the tags.
So I think probably the best way to use tags is to think in terms of how a user might search. First, I just tested it and spaces don't work, so I recommend squishing multi-word tropes into one like "slowburn" as one word if using that term, though some works seem to have both "slowburn" and "slow_burn".
It might be worth looking at trope names on stuff like Ao3, where tags are more widely used, for inspiration on what to add... but at the same time, since tags aren't very visible, I'm not sure whether it occurs to people to search tags on Tapas at all. It's pretty hard to work out how anyone found a title on Tapas, so there's no way to really test effectiveness of tags. Can't hurt to add some, but I wouldn't say it's as important as a means for getting readers as having a good cover, blurb and banner.