For those words, there are different reasons. ゴミ and カバン are written in Katakana as their kanji (塵 and 鞄) are not included in the joyo kanji list.

In this blog post, I explain why ゴミ is written in Katakana:
https://epjapanesey.com/muskas-quote-look-the-people-are-like-rubbish/

As for イス (椅子), the kanji “椅” was registered in the joyo kanji list in 2010 but until then, we’ve been using イス for a long time so it’s still written in Katakana even though it’s in the joyo kanji list.

Good question, Chris-san!
There are various reasons when we write Japanese words in Katakana, but in this case, it’s to soften or lighten the impression that the Kanji word has.
化け物 in Kanji shows the literal meaning “things transformed” which gives you a creepy and serious impression, but Katakana makes it look more casual. I used バケモノ in my manga so that it looks more comical and funnier than 化け物. In other words, on proper books or blogs which takes monsters seriously, they would use 化け物.
For another good example, loan companies use ローン instead of 借金 in their ads to make people think it’s something easy and casual to try.
Hope it’s clear!

Thanks for your comment! We have received the similar request before so we added printing buttons so that you can print it out and check the vocabulary on the sheet. But we will consider changing the position of the vocabulary as well. Thanks again for your feedback!