Is Bombing viable at the 3* and 4* levels?
I've read through some Bomber threads and looked up youtube videos on bombing. I've got the 2* bombing equipment, and while I'm really interested in end-game bomber sets and bombs, they seem lackluster at lower levels. Every crown counts at this stage, and I'm wondering if it's going to be frustrating for me trying to go mostly full bomber through 3* and 4* equipment.
Should I stick to something more generic and easier for now (i.e. make Wolver series sets) and wait until end game to craft my 5* bombing equipment? Or is the 3* and 4* bombing equipment viable for full-on bombing all the time? Any advice (on this, on on sets) is appreciated, thanks.
Don't know about PURE bomb in T2, at least for solo. I had a calibur + fiery vaporizer line with bomber armor and it worked pretty well. I definitely found myself missing my fire bombs when I was playing on my sword user later, but there's some things that are just plain easier and time-saving with a sword. Just try to avoid fire-type stratum with that setup or pick up a 3rd weapon slot.
Of course, in a party, bombs are fantastic, but I usually go solo anyways =) power to the bombers!

Then don't go full bomber. You need a sword, a gun, and a bomb, and you need to know how to use all three.
Use what you love - if you like bombing then keep at it. For a long time I used a 4* Master Blast Bomb and a 4* Fiery Atomizer worked great... and that was before the lower star haze bombs got a radius increase. The really huge radii are useful for arenas and danger rooms, but for most of the clockworks a 4* can fill a good portion of the passage.
You will want a weapon to complement your bombs though. The Calibur is a great starting weapon for a bomber since it does normal damage it'll only take up one weapon slot while being effective vs all enemies. It goes up to 5* so you can use it forever. Also the charge attack combos nicely with the freezing line of bombs.
Generally speaking, the "off-specs" of bombers and gunners are a really inefficient way to play the game. I wouldn't recommend doing it until you're self-sustaining with at least one 4-5* set. Virtually everything in the game is faster and easier with a sword; there are very few situations where you'll say to yourself, "A bomb or gun would be better here!".
It's certainly viable, you'll just progress significantly slower.
That said, it's a game so play the way you most enjoy playing. Just know that if you group, people usually do not want bombers to use any kind of blast bomb because of the huge knockback and explosion that obscures the screen.

If it's a pierce damage bomb. I've often used the snarbolax 5* bomb to solo throughout entire levels of deconstruction zone gremlins and wolvers' dens even in the 6th stratum (levels 24-28). It's so easy it borders on boring!

I spent a lot of time with a Master Blast Bomb as my primary weapon. It was very effective. Of course the upgrade to Nitronome was nice, but the Master Blast was certainly viable, even into tier 3. I also used the freezing vaporizer and fiery vaporizer at 4* for quite a while to good effect before finding the 5* recipes to upgrade, and now those bombs are even better at the 4* stage than they were when I had them. The 3-4* armor is nice, too, although now that piercing defense is more important I'd consider mixing and matching (demo helm with wolver or gunslinger coat, for instance) or doing what I did and buying a royal jelly band to supplement your defenses.
Piercing bomb is nice, but I'm perplexed as to why the guy above me recommends it for decon zones. Piercing is terrible against constructs, and decon zones have gun puppies and mechaknights and there's no bonus using piercing on gremlins. The primary blast bomb line is better for deconstruction zones, as is a fire bomb (as long as it's not a Molten Mayhem level, of course). Piercing is good against beasts and fiends, so useful against alpha wolvers and devilites, if you can charge it fast enough to use it before they hit you with a chair.
its viable, just inefficient. If your goal is to be a bomber, but would prefer to have an easier time in the early stages, then craft your bomber armor and bombs, but keep a generic jack of all trades sword with normal damage like calibur, cutter, or troika series.