Should I have undergraduate class papers on ResearchGate?
From experience, both as a member of ResearchGate and as a postgraduate, I would say the main question is more about worth than about acceptability. 

Sounds like you did the right thing when you posted these papers the first time, with characterizing them as undergraduate works, therefore managing expectations.

You feel these papers may be of interest to others; fair enough. Statistics of opens and reads, and any message you get about them could be a good indication of how worthy they are to peers. If nobody gets to see them, you might as well keep them to yourself. 

Over 100 reads for an undergraduate paper sounds rather cool, and might give you an interesting clue about the popularity of your paper topic, and might encourage you to work further around the same topic for further research and papers.