Jigglypuff is seriously not THAT bad; you guys are talking like she's useless or something. She's definitely a low tier character, but she can win and her strengths do matter to the game as a whole. You could say her strength against it "helps" her in the same way that Falco's chaingrab does. By not being effective against her but being effective against some other characters, these things make her better by comparison to the rest of the cast (presuming they hurt more characters than they directly help, which is probably true for both planking and Falco's chaingrab). Her own abilities to directly play stallish ledgegames are positive for sure as well.
Anyway, the difference from wallbombing is simple. Let's assume it's being done on Fountain of Dreams back in melee so we can easily represent the position. Peach is under the stage very low and can stay there basically forever. How many characters can attack her down here? Ness obviously can. Jigglypuff and Kirby and probably make it down there, try to hit her, and make it back. Another Peach could start wallbombing even. Everyone else doesn't really have a chance (remember you have to do an aerial attack extremely low and then recover so the elaborate recoveries that a few characters like Mario can make from that low won't be working). In order to even try to hit her, they have to commit to literal suicide. Ness and Kirby weren't really tournament viable anyway (unless Ness is on Corneria!) so basically the whole metagame is now Jigglypuff and Peach with Jigglypuff being at a pretty big disadvantage. This is in sharp contrast to planking. With planking, most of the cast have very reasonable counters. The balance of the game isn't going to change a whole lot either way. Even the FEW characters that don't have practical counters only have to put themselves in a position they are very likely to get hit to try, not put themselves in a position where they are absolutely guaranteed to die even if they hit.
For that matter, even if planking were something that most of the cast had serious trouble answering (it isn't), it would still be worlds better than wall bombing. If you attempt to dislodge a planker, what are the possible outcomes? You could hit the planker and be safe, trade hits with the planker, and get hit by the planker. That's one winning, one potentially neutral (trading is situationally good and bad), and one losing. When you approach a wall bomber, what are the possible outcomes? You can fail to hit Peach and then die or succeed at hitting Peach and then die. That's one option that's losing and another that's probably losing but has a small chance of being neutral. Even if planking were a ridiculously strong tactic, it doesn't risk death unless the player really screws up or dies in the way getting hit normally kills (high damage, recovery situation, etc.) and has a clear possible winning outcome for the person playing against it. With wallbombing, death is not only likely but absolutely certain if you are using any but a VERY small number of characters and even the best possible outcome is still probably losing for the person playing against the tactic. They are seriously not analogous at all.