Traditional sushi rice ("meshi") is defined by both the stickiness of its rice but also its lack of gumminess. Each grain of rice is supposed to remain distinct. The final component of sushi-meshi is of course the application of vinegar, which is supposed to evenly coat the outside of every grain. Proper application of vinegar is not possible unless the grains remain distinct. Therefore, if the rice is overcooked or cooked too slowly, water will have more time to penetrate and the starches will have too much time to swell, causing it will turn into a sticky mess with indistinct rice grains. Furthermore, basmati rice can't be used for traditional sushi-meshi because it doesn't have enough amylopectin (the starch in rice that makes it sticky/"glutinous"). Glutinous rice (which has 100% amylopectin and virtually no amylose) is used for ketupat but isn't used for sushi-meshi, since it is too sticky.
There is nothing culinarily wrong with using a slow cooker to make rice; if it tastes good, eat it! In fact, a quick web search reveals many recipes. Here is also an answer to a relevant question. But I don't see a way of making sushi (for which there are relatively strict traditions and definitions) using a slow cooker.