I'm not so familiar with the C64, but Monkey Island did indeed use graphics mode on all the 16/32 bit systems it supported - PC graphics cards had their video memory on the card (same as they do today), so saving memory by using text mode didn't make sense. The only problem with that was that the CPU had to be used for any processing of the video memory, so especially scrolling the whole screen was sometimes a bit slow with weaker CPUs. The Amiga and Atari ST didn't have a dedicated text mode.
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I'm not so familiar with the C64, but Monkey Island did indeed use graphics mode on all the 16/32 bit systems it supported - PC graphics cards had their video memory on the card (same as they do today), so saving memory by using text mode didn't make sense. The only problem with that was that the CPU had to be used for any processing of the video memory, so especially scrolling the whole screen was sometimes a bit slow with weaker CPUs. The Amiga and Atari ST didn't have a dedicated text mode.