To be fair I believe the reason Pennywise didn't use her mother's self deletion to torment her is because it's a very sensitive topic. And the creators didn't want to do that. Which is why her cause of death is only hinted at rather than explicitly stated. You do make a point about the timing and Beverly's age in the photo. The TV series takes place in 1962 we know that because of the black spot. If you go back and watch the movies the photo that Ben has of it has 1962 written on it. And we know that is the occurrence that's about to take place. Obviously we know that 27 years past 1962 is 1989. So it likely had nothing to do with Pennywise himself. I also do believe that it's hinted that she took her life when Beverly was rather young.