Although it deals decent damage and high knockback; Bowser bomb has a predictable trajectory (straight down), high ending lag and weak priotity that make it a very unsafe move to use as a regular attack.
However, the Bowser bomb has some unusual properties, notably weird momentum cancellation, a weird, 2 hit trajectory when grounded and relative high damage (and the fact it will remain fresh) which makes it a very viable option in some situations and a good suprise move.
Use it to:
-Get to the edge and ledgestall. No need for an explanation.
-Break shields. Jab+2hit Bowser bomb= broken shield. If the opponent shields your jab, you should follow imediately with a gounded down-B. No one expects it, and you are then free to follow up with a charged fsmash. Don't do it more than twice per match though: it works only because the opponent won't think of dodging it; and only that.
I also use it to punish uairs. Yep, it sounds stupid, but it actually works sometimes. Once again, don't try it on link, and only use it in the right circumstances. Bowser's down-b seems to cancel all downwards momentum, so either you do i a tad early and trade hits with the opponent or you time it make the opponent whiff his attack (its predictable though). I never dared to use it like that on a good opponent though, so I don't really know if that use is tournament viable or not.
Overall, Bowser bomb is your high-risk/miscellaneous-reward move that relies on your opponent's cluelessness to land and as such is extremely situational but has the potential to change the momentum of the battle. I usually use it to break a shield (if I have the opportunity) as soon as possible to scare the s*** out of my opponent each time I jab his shield and a second time for a "free" kill with Fsmash.