To be honest, I disagree with the notion of merely blocking Robin's projectiles. For a lot of projectile characters it's not so bad of plan as long as you don't let yourself block compulsively or get baited into a predictable rhytym.
However, while Bowser's other options aren't as safe as shielding I don't think they should be underestimated. There will be times where you will want to short hop or possibly even full jump. There are also times where you want to just spotdodge or roll.
Let's look at the Robin example; the majority of the time, Robin will throw some multi-hitting projectile at you. If you simply shield, all you're going to do Is give Robin stage control and/or damage for free, especially off of arc fire. You can't do much of anything after blocking an arc fire/thunder other than either sit there and wait or attempt to roll back if you angled your shield so that the projectile won't just hit you out of your roll for free.
None of these are good options and basically lead to you waiting till arc fire runs out so you can start playing. Robin knows this too and will just pull a rosa by stalling hard when books run out. It's not a very fun or efficient way to play the match-up. So instead why not simply jump or roll?
These options are more risky in general than shielding but you get better rewards when using them correctly. One thing to note about Robin's projectiles is that they are all laggy on cooldown. Also, none of them can hit you during a full jump so you can actually do a neutral full jump when you feel an arc fire is coming and that should give you a free aerial if you follow up accordingly.
However, another important thing about utilizing the jump is that you force Robin to consider it before deciding on a course of action. If you're going to be on the ground 100% of the time you've already given up one possible layer to your mix-up. Which in turn makes it easier for your opponent to pick strategies which cover your options.
Rolling provides much of the same benefit, if you roll correctly against one of Robin's projectiles, that's typically going to be a free klaw, you might even be able to get a turn-around grab or down b as well. This also, if employed correctly, forces Robin to adapt to you and try to figure out when you will roll. Then suddenly Robin's train of thought might go from "Well, he's going to either shield or run away from my arc fire but either way it's safe!" To "Well he's mixing me up with these jumps and rolls, I'm going to need to get a read on those options so I can punish them."
Now don't misunderstand, I'm not saying to stop shielding and just spam jump and/or roll the entire match. You still need to play neutral and predict your opponent's options in order for these strategies to work, and Robin can still easily punish either option. However, simply making Robin consider those two options at all gives you an edge because now you have a mix-up.
You might even get a situation where Robin predicts you trying to roll and tries a roll punish instead of playing the usual spacing game with arc fire, which would lose to your roll.
Now one thing I should add is that you need to be wary of Thoron. That's definitely one projectile you'll generally want to block since that's the most reliable method of dealing with it. That being said, jumping over thoron is doable and lets you close the gap while Robin is stuck in the animation but is also more risky because you might get hit mid jump or simply jump too early and be forced to land against Thoron, which kinda sucks.
Of course, Thoron is slower than human reaction so if jumping over it consistently on reaction is possible then jumping gets a lot better, but is still riskier than shielding unless your shield energy is critically low.
Long story short, the "best options" may not always exactly be the best options. Mix-ups are powerful even in the simplest forms, apply them in as many ways as you can.
Hope this helps!