Trump made a similar promise at his own rally in the city in June — though neither he nor Harris are likely to be able to fully do that without actions from Congress.
I'm thinking in the political game theory, it makes sense because it's going to force Republicans to either support it or oppose it, either of which will make them look silly as trump supporters.
The revenue losses would need to be made up in another way, which would ideally be closing loopholes and more dependably taxing the wealthy. That is assuming that the segment of revenue from tips is even substantial enough to matter.
Anyway it's going to force the Trump camp to respond. And pretty much every time you do that it provokes more unforced errors from them because they are clueless.
Just another note, Biden has also mentioned he would sign this into law. This would be even better for Dems than Kamala's promise. For an election cycle, it would basically delete any value of Trump's promise to tip earners.
I've seen "no tax on tips" on Trump billboards on my area. Clearly Trump thinks it's an important issue. Having made them waste money would be a hilarious win, also preventing undecided voters from turning out for Trump on this single-issue is a clear strategic design.