Yes, it seems to be the case that sports viewership numbers have increased since Murphy (which allowed states to legalize sports gambling in the US). COVID of course made this a little complicated to measure, but viewership has increased by several percent in the years since.
It also increases the viewership of games during blowouts, which obviously makes their eyes more valuable to advertisers (who otherwise would see a drop in viewers when the game is no longer remotely in doubt).
The exact numbers are hard to say, because so many confounding factors exist, but for example the NFL ratings rose 5% in 2019, the first year of legalized sports betting (in 13 states by that point). That was pre-COVID, so avoids some of the other confounding factors there, but of course there could be other factors affecting it.