The point of the episode is quite simple, Bonnie is a main character in the group, this is a development episode for her, her/Clemont's Dedenne, and also development for her within the main group. That alone should take it out of the realm of filler, but alas, some of you only have one character you want to see development for, so anything else just won't cut it.
You can't call character development something you haven't even seen the outcome of. Dedenne getting lost and Bonnie trying to find it is not progress. If the absence of the rodent causes her to change or introduces a new character trait that we haven't seen, then we can confirm growth or expansion of her character. In the meantime, I suggest the "character development" card is properly used.
Getting focus or screentime does not necessarily mean development.