My bet is that you will have to change in Braga. If you go to the Rede Nacional de Expressos website, http://www.rede-expressos.pt/, and click on "horarios", you'll see you can go Barcelos to Braga on one of four buses, 6:30, 8:30, 13:30, and 18:30. It's about a half hour ride. Only the later two buses are daily, the first on is only M-F and the second one doesn't go on Sunday.
Then, once in Braga, it's another half hour to Ponte de Lima. It looks like (though I find this surprising) there is only one bus a day from Braga to Ponte de Lima, leaving at 6:30 pm daily, which is the same time that the latest bus arrives from Barcelos. So the latest you can leave Barcelos is on the 13:30 bus, which means you will have a layover of at least 5 hours.
BTW, I think you will easily find ways to entertain yourself in Braga for a day. It's a lively provincial city with a nice old core, some very nice cafes, plazas, fountains, etc, along with its very old cathedral.