I'm not sure what this "tanky top" meta is that you're referring to.
Riot has continually nerfed defensive itemization for bruisers and tanks alike. Trundle has to build pretty tanky to do his job, and is countered by ranged APs in lane (also Quinn and Vayne). As far as I've seen, on pretty much every map, people in mid-higher MMRs are running a lot more ranged magic damage champions, even in top lane. This makes sense considering how good AP itemization is currently, the lack of strong MR items, how much utility most ranged APs tend to have in their kits (meaning they scale well into late game even without getting fed early), and the comfort zone of being able to farm/harass from range.
Trundle is good against other bruisers and tanks, but pretty terrible against most ranged champions in lane, especially AP. You have to be really good with the Troll King to win against his direct counters (which are currently many).
When bruisers/defensive itemization as a whole are good again, you'll start seeing Trundle more often.