Does a Nikon D3100 take good pictures indoors?

I have heard that a nikon d3100 doesn't take great pictures indoors without a 35mm lens. If I get a 18-55mm lens will it take ok pictures indoors or will they be bad? And how bad? I don't need it for professional use yet.

Depends on how much light you have.
The D3100 is an entry level camera, not something a professional would use.

Eh? There's a 35mm in the 18-55mm lens.

It's just that the D3100 was heavily marketed as a beginner's dSLR that's why so many complaints come out. Most D3100 users don't know how to use a dSLR. If you know the basics of photography, you can use the D3100 to take pictures anywhere, anytime and get good results consistently. It doesn't matter if you intend to sell the pictures or not although you can use the D3100 professionally if that is what you want. The key is knowing what to do with the camera. Picture quality depends on the photographer. The camera and lens are just tools.

How long is a piece of string?

There are plenty of lenses that will work well with your camera indoors, not just a 35mm prime, and an 18-55mm with an f/2.8 maximum aperture is one of those lenses. Add a flashgun with bounce and you'll do better than with the inbuilt flash. Use multiple flashguns set up as control and slaves for even more control.

So it's really about as good at taking pictures indoors as you care to make it!

If you know how to properly operate the camera it'll do a passable job. That applies to any camera that's not using flash indoors - passable. But to get passable you must know how to work the controls. Other wise you'd might as well save some money and buy a cheap pocket toy.

Whoever gave you that advice about the 35mm probably meant to explain that a lens like the 35mm f/1.8 AF-S lens would be much better in low light than your 18-55mm kit lens. In fact, the 50mm f/1.8 G lens would be equally capable, but a bit more expensive.

The D3100 can work well indoors without flash, if there's enough room light, you've selected a high enough ISO, and are using a slow enough shutter speed. The reality is, that a f/1.8 (or the much more expensive f/1.4) lens will allow you to use a lower ISO (sharper photos) and a faster shutter speed (less motion blur).

Look here for some photos taken with a Nikon D3100 and the 35mm f/1.8.

By the way, I use that exact same lens on my Nikon D300 and find it to be my standby when the light is most challenging.