Disclaimer: I'm not an expert on motorbike frames. I do know however that with push-bike frames, adding gussets like this without careful design can make things worse. It stiffens up a specific area of the frame, so it means any load is applied, the stiffened area 'attracts' more load. As you'd expect, at critical loads, it's normally the metal around the welds of the gussets fail first. If the plates have been well designed, you get a higher ultimate strength....
...but the plates can focus loads to a weaker area of the frame that weren't designed for it and failures can occur at lower loads than before. And that's before you consider the weakening and locked-in loads than you can accidentally create from a poor weld job.
This is probably fine. But I'd want to see some evidence that it does actually help.
