No, not ground. Not unless the keyboard decoder has 96+ pins, one for each key.
Each key connects two wires, a row and a column. the encoder chip in the keyboard setects which two wires are connected and then sends a keycode to the console.
Attached is an example design using 6mm tact switches and two headers.
On the bottom (blue) layer every button is in a column is connected on one side to the same pin/wire in the column header.
On the top (red) layer every button in a row is connected on one side to the same pin/wire in the row header.
The decoder has pins that connect to each row/column.mc.pp.seDreamcast programming - Keyboard
shows us that a Dreamcast keyboard has 96 keys in six rows and 18 columns; however there are 104, and 88 key versions that will have a different matrix layout. the attached example isn't perfect, keyboards are rarely square grids, and keys are not always exactly one gird wide; so if yo use this you will probably need to relabel some keys; or look at the original membrane to see what key connects what two wires.
Pro tip: most tactile switches/buttons connect the two pins on one side together; so it is possible to run a signal in one leg and out the other. This makes making PCB's cheaper, and the attached example makes use of this.
Only the bottom layer is needed if all of the solder joints and traces are good.
In case you do decide to use it, I've deliberately designed the two layer version to have parallel circuit paths (of equal length) meeting each switch in two places so that it will keep working if something breaks.
edit: attachment refused, "Sorry, the board attachment quota has been reached.
" it says.
Sorry, but Google has corrupted it; it can nolonger be used to produce a PCB directly, and is only an illustration.
wait, tried again, it is now downloadable. docs.google.com2010.09.11.pdf