To echo what Hope said, pin values are pretty subjective, since we're all searching for different things. (Except for the pin snobs who just want anything rare/popular for the sake of bragging rights...) I personally try not to get too caught up in the supposed value of pins. It tends to spoil the fun if you're busy trying to work out some mathematical formula for every trade, or worrying about whether you're throwing away a totally hypothetical better offer that might never even happen. If your gut is telling you that the trade is unfair, or you've got some super rare trader that's probably worth being picky with, then you should certainly give it careful consideration before doing something you could potentially kick yourself over. Otherwise, just trade for the pins that will make you happy, period. That's the point of the hobby, after all.
That said, I did find 2 current eBay listings for Mochi (I think Hope was using different search terms.) They're priced at $150 and $160, while the 3 Bing Bong listings range from $70-$90. However, those numbers really don't mean anything; just because someone is asking for $160 doesn't necessarily mean anyone is willing to pay that much. The best way to estimate the actual going rate for a pin is looking in the Sold Listings section. I see that since October 1st, 2 Bing Bong PTD's have sold - one for $39 and the other for less than $65 (best offer was accepted.) Since November 21st, 2 Mochi PTD's have sold - again, one for $39, and the other for less than $50 (another best offer.) So it looks like they're roughly equal in monetary value. Because you're looking to trade rather than sell, something else to consider is how long you've had the Bing Bong PTD for and how many trade requests you've received for it in that time. If it attracts a lot of interest, or if you've only just added it to your traders, then you might consider waiting for a pin you'd prefer (how high does Mochi rank on your wants list? It sounds like it's not a "must-have.") If not, it sounds like a perfectly fine exchange, imo. Disney Cats are a reliably popular pin theme and it's a safe bet that, should you later regret the swap, Mochi would make an excellent trader in his own right. Hope this helps with your decision and future trade assessments.