I think you have a very biased reading of the show.
Comparing Froppy to Uraraka or Iida is unfair to the character, they're part of the main cast, it's like saying Bulma is unimportant compared to Vegeta or Piccolo.
Comparing Froppy to Momo makes more sense, but I think your memory is selective.
You said she never do anything on screen, but she was part of the action during the final event of the first season and during the first half of the third season.
Sure, we didn't see her going head to head with a villain, but we didn't see Momo doing so either.
You feel Momo is more important to the plot, but in which way? She did a device to track Nomu and then what? She went with the boys and... did nothing?
And the show is still going on, Froppy isn't part of the main plot... yet it seems to me that she is one of the few students that will face Overhaul in the next episodes.
Sorry for the anime sperging.