By "least successful", I would define it as within five years or so. This isn't counting Miskel Spillman (https://en.wikipedia.org/wiki/Miskel_Spillman), people who tragically died shortly after their guest hosting stints (https://www.thewrap.com/snl-hosts-list-all-time/) like Strother Martin, Chris Farley, or Chadwick Boseman, or hosts (https://en.wikipedia.org/wiki/List_of_Saturday_Night_Live_guests) who were admittedly already in the twilight of their careers like Desi Arnez or Betty White. I also don't know if political and sports types should count or not since they weren't really going for Hollywood careers anyways.

Regan's bratty kid would be the correct answer.

Are there (https://poe.com/s/rozQJBRiimI8oR3A4Tm7) any hosts (https://www.google.com/search?q=Are+there+any+hosts+whose+careers+went+downhill+shortly+after+appearing+on+%22Saturday+Night+Live%3F+By+%22downhill%22%2C+I+would+define+it+as+within+five+years.+This+doesn%27t+count+hosts+who+tragically+died+shortly+after+their+guest+hosting+stints+like+Strother+Martin%2C+Chris+Farley%2C+or+Chadwick+Boseman+or+hosts+who+were+admittedly+already+in+the+twilight+of+their+careers+like+Desi+Arnez+or+Betty+White.&oq=Are+there+any+hosts+whose+careers+went+downhill+shortly+after+appearing+on+%22Saturday+Night+Live%3F+By+%22downhill%22%2C+I+would+define+it+as+within+five+years.+This+doesn%27t+count+hosts+who+tragically+died+shortly+after+their+guest+hosting+stints+like+Strother+Martin%2C+Chris+Farley%2C+or+Chadwick+Boseman+or+hosts+who+were+admittedly+already+in+the+twilight+of+their+careers+like+Desi+Arnez+or+Betty+White.&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCjQxMDAyajBqMTWoAgiwAgHxBXwMZ5tRa931&sourceid=chrome&ie=UTF-8) whose careers (https://x.com/i/grok/share/arg4DY9DIXhMHJ3pZe2mMAYM0) went downhill (https://chatgpt.com/s/t_69104467d9848191b31ad0c17c221fea) shortly after (https://www.perplexity.ai/search/are-there-any-hosts-whose-care-Id7Z.n3uTBm.aDCC1VMkwQ#0) appearing on Saturday Night Live?