Is it feasible to visit Malaysia on an 18 hour overnight layover in Singapore?

Yes, but I would not recommend it, especially given the short time you have.

Singaporeans frequently go on a day-trip to Johor Bahru (or "JB" as they simply call it), which is just across the Causeway (the bridge linking the two countries) to the north of Singapore. Sometimes they even go slightly further afield for a day trip. So yes it is perfectly feasible.

Food. Singaporeans and Malaysians like to argue over whose food is better. IMHO, the food in both places is superb and has fantastic variety; and to the Westerner visiting only briefly, it will be indistinguishable.

Shopping. Shopping in Singapore is better, if more expensive. Singaporeans do cross the border to do their shopping, but mostly to get quotidian stuff (like chewing gum which cannot be sold in Singapore).

Culture and feel. Malaysia definitely has a different feel from Singapore. But like many cities in Malaysia, Johor Bahru has a large Chinese population (close to a majority). Moreover, JB is amongst the richest cities in Malaysia (though still visibly poorer than Singapore). So while it is certainly different from Singapore, it is less different than you might imagine.

You land at Changi at 2:30pm. Best case scenario: Clearing immigration + collecting your luggage takes only 30 minutes; taking a taxi to the border is another 30 minutes; clearing immigration there and getting into JB proper is another 30 minutes. So best case scenario you're in JB at 4pm. So you have late afternoon and evening in JB. Then you have to get back to Changi no later than 8:30am the next day. So altogether it just doesn't seem to be worthwhile. It'll just be exhausting running about for a low return. Better to just stay in Singapore.

I really think you should reconsider the easiest option. Take a plane to Kuala Lumpur (flying time 1 hour 15 minutes give or take) - and there are more than 40 daily flights.

From Kuala Lumpur Airport, take the KLIA Ekspres high speed train into Kuala Lumpur city center (travel time, about 30 minutes).

So in about 2 hours you'll be in the heart of Malaysia with food and shopping to satisfy any sort of appetite.

Absolutely. Upon arrival at Changi airport, take a taxi directly to the Malaysian Causeway/Johor Bahru border crossing. If you have ever crossed the US border into Tijuana for example (and/or vice-cersa), then you should expect a similar experience. The S'pore side is like the rest of Singapore - clean, tidy, well maintained, and efficient. It takes just a few minutes to walk across. As soon as you step into JB, there is an sharp contrast caused by abruptly changing from a 1.5th world to a 2.5th world environment, but the reason I mention it is because you should use this to your advantage - go get a cheap motel room close to the border, put your bags down, go explore. After a few hours once you get back to the room, take a cool shower, cross back over to Singapore by bus or by foot, grab some Chicken Rice at the nearest Hawker Center, then take a nice cool bus or MRT ride back to the airport.

imho however, any time spent in JB that could be spent in Singapore is not time well spent. There is just no compelling reason for the average layover tourist to make the trip when in Singapore you already have the best of everything at your fingertips. At least in Tijuana you can get a Poncho, 50 x 5 cent individually wrapped TicTacs from the friendly local kids, and a 6 month supply of (your expensive prescription here). JB simply doesn't have these type of offerings.