The t-shirts wouldn't bother me and I've seen much worse, on a Jet2 flight as well - we had a horrendous Friday morning flight to Mallorca a few years ago.
Half the passengers were on stag or hen weekends and they were charging up and down the plane, pissed with t shirts saying things like 'Shazza's hens do Magaluf 2015 on the front' and 'Brenda likes it up the arse' on the back.
I expect that the group referred to in the OP got extra rowdy when they were asked to cover up their t shirts, which is why they got thrown off. Also, I think jet2 have had quite a problem with having to divert for drunk and disorderly passengers, some of whom have been heavily fined or even imprisoned.
The atmosphere on planes to Mallorca on recent flights I've been on has been quite offputting - we had a similar experience to the above going from Liverpool with Easyjet last year, and I've come to the conclusion they should put separate planes on for people going to naice Mallorca so they don't have to spend 3 hours in close contact with drunken idiots going to Magaluf
.