Discrete doesn't mean good. There are discrete graphics card out there that perform as well as or worse than an integrated GPU.
The Radeon Pro 555 is not remotely a half decent discrete graphics card. If you check the benchmarks here, you can get above 60 FPS in some games only if you drop the resolution to 1280x720 and use low settings. The Macbook Pro 15" has a 2880x1800 resolution, which is 5.625x the pixels. Pixels to required GPU power isn't exactly a 1:1 ratio, but to make it easier, you can just say that the GPU would need to be about 5.625x stronger. That would only get you 60+ FPS at low settings at 2880 x 1800, so you would require an even better GPU than something that's 5.625x stronger than the Radeon Pro 555 if you want 60+ FPS and high settings.
So yeah, it's pretty really bad.
If you just want 30 FPS low settings, drop the resolution to 1280x720. Dota probably also has better performance on Windows, so bootcamping could help a bit.