Wouldn’t Jane need a mask for her land (Crypts and Helium) because of the properties in Helium? I don’t see why she was less deserving off a mask than Dirk and Jake, especially considering all the time she spent on her land.

Jane’s Land’s atmosphere is not filled with Helium, rather there’s vents that produce it and Balloons that float up when filled with it. As such, the amount of Helium in the air is likely not too overwhelming, in fact since it’s lighter than air there may be no noticeable traces on the ground, and while long exposure may get her dizzy, as long as she doesn’t shove her face on one of the vent thingies she should be alright, albeit perhaps with a squeaky voice.

Jake only wears a Mask in Dirk’s Land, so it’s not both Jake and Dirk- Just Dirk, and in his case, the atmosphere of his planet IS filled with Krypton. But only at the lower levels- Since Krypton is heavier than air, we see him wear the mask at the bottom, but not at the top, it sinks down. Krypton is not toxic, but in high concentrations it can be suffocating because of the lack of oxygen, which is why they need masks.