No, they are far laggier...
Mobs do not take much RAM.
A mob takes far less RAM than a single chunk of blocks.
Mobs are only "Laggy" when you consider that they're basically 90% CPU and GPU calculations, and you have so many that the computer can't keep up with the queued calculations.
A powerful computer may have very little RAM, and a computer with a lot of RAM may not be very powerful, one could be both, and one could be neither. RAM is a completely separate issue from CPU, GPU, and motherboard/bus speeds.
In Minecraft, get Optifine for the enhanced view distance, graphics options, and overall performance boost. Spawn 2,000 mobs in a chunk, check your frame rate. Should be smooth. Keep adding more until the game bugs out or crashed. Frame rate should still be relatively high, or at least as high as it can get with that many polygons on screen (or still being rendered offscreen). If your graphics card is powerful, and your view distance is set to Short, then you could probably keep adding them until the computer itself starts chugging.
Get rid of the mobs.
Now, set the view distance slider to Extreme+1024. You will NOT get that much loaded, unless you have a lot more RAM than anybody has any business having. Walk in a straight line. It'll take a while to load the chunks in the direction that you're walking, while chunks from the area you're leaving will be unloaded pretty quickly, since you're trying to load so many at once (due to your view radius choice). You'll also see a massive framerate drop because the GPU is trying to render a few million blocks.
Also try Starsiege: Tribes. 100megabytes, free download these days (technically - Dynamix/Sierra gave up enforcing anti piracy stuff after the Fox incident). Set it to software rendering. Join a ModX server. Set the object spawner to "newdam". Aim FAR FAR AWAY. Spam click to spawn this massive interior object until your graphics start dying - that is, you'll see every texture in the game get painted over the terrain, sky, objects, etc. and it'll flash as it attempts to keep all of the texture info loaded for each individual object, despite the software renderer's hard memory limit. OpenGL doesn't have this problem since it's basically got infinite scaling capabilities compared to older games, but it'll get the idea across.