This is my serious face. Oh, and I Call.
In some ways Bubbles's lightheartedness is similar to Pinkie Pie's -- and they are both The Laughter after all. But Bubbles is also the Beast Tamer of the team, and that does take emotional fortitude and compassion -- thus, like Fluttershy, Bubbles may be the only one capable of taming a chaotic element. Plus, she does speak Squirrel.
Blossom being the Mind, Buttercup the Body, and Bubbles the Soul is a reflection that, while each has her own specialty and strength, they work best when they're working as a team, thus their most powerful techniques have always been team techniques -- yet BeeAre was careful to make sure to split the team very quickly. And even if Buttercup can beat Sam and get to Blossom before Bell goes berserk over the great on Gir, the team will still be incomplete. Blossom may be able to come up with a strategy for her and Buttercup to double-team on Bell, but the team will still be missing its Heart -- and we know Bubbles doesn't rejoin her sisters before the bomb goes off. I find it unlikely anyone but the 5 already there (including Gir) will play any part in this battle.
It also seems curious that Bell, being White (Blank) is the sum of Blossom (Red/Pink), Buttercup (Green), and Bubbles (Blue) instead of Black (All-Consuming Sum).
Yes, 'bel' does meant 'battle.' Just like 'Cantus Bellax' means 'Song of Battle' or 'Battle Hymn.' It's from Negima.
Sometimes when an author is writing, the characters may seem to take on lives of their own and do things seemingly of their own volition, making things happen that weren't originally planned. This means that the author may not know everything. But I do not think they dislike seeing their readers enjoying their works and speculating about what will happen next. If they did, BeeAre would never come here. Certain writers are notorious for making sure a speculation does not, however, come to pass. But if it's part of the overall story arc, then of course it would happen anyway: it's part of the Plot.