Is it just me, or is Tough Skin mastery underwhelming?

Yes it's shit

most of the resolve tree is shit.

there are a couple useful utility masteries like tenacity, but the defensive tree doesnt actually grant any noticable defense.

note how the only % damage reduction mastery doesnt even benefit the guy who picks it, but essentially gives carries a second keystone.

its just you.

its junk. worthless.

It is not underwhelming. I almost always get it when I'm playing top and going into the resolve tree. It's a poor man's Doran's Shield

It's actually very common, don't know why people are saying it's worthless. Just look up the most popular masteries used on bruisers/tanks like :

• Irelia: http://na.op.gg/champion/irelia/statistics/top
• Poppy: http://na.op.gg/champion/poppy/statistics/top
• Darius: http://na.op.gg/champion/darius/statistics/top
• Malphite: http://na.op.gg/champion/malphite/statistics/top

Most people, at least from Plat+, take Tough Skin for top lane. Explorer is more common on supports and junglers.

Or just look at the masteries of the best top laners:

• Smeb: http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/235296945
• Duke: http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/235297029

Look for their Irelia, Kled, Poppy, Gnar, Rumble games. They always take Tough Skin over Explorer.

As for the Resolve tree being "shit"? Kind of hard to make a case for that when many tanks still opt to go for Undying Grasp.

It helps on the first jungle clear. But yeah, it's not overly impressive.