Infinite loops seemingly not working in NASM?

The problem isn't actually the loop itself. What the loop is doing each iteration is the problem. Some issues and observations I have are:

• Since this is a DOS COM program you will need an org 100h at the top since a COM program is loaded by the DOS loader to offset 100h of the current program segment. Without this the offsets of your data will be incorrect leading to data being read/written to from the wrong memory locations.

• You have a problem with mov al,pixelcolor. It needs to be mov al,[pixelcolor]. Without square brackets¹ the offset of pixelcolor is moved to AL, not what is stored at offset of pixelcolor. The same goes for pixelx and pixely. Your code prints the same pixel color (red in your case) to the wrong place² on the screen repeatedly. This code:

    mov ah,0ch
    mov al,pixelcolor
    mov cx,pixelx
    mov dx,pixely
    int 10h
    inc byte[pixelcolor]
  

  should be:

    mov ah,0ch
    mov al,[pixelcolor]
    mov cx,[pixelx]
    mov dx,[pixely]
    int 10h
    inc byte[pixelcolor]
  

• It should be noted that the resolution of the timer by default will only be 18.2 times a second (~55ms). This is less resolution than the 1/100 of a second you are aiming for.

• Some versions of DOS may always return 0 for the 1/100 of a second value.

• Use of the BIOS to write pixels to the screen may make coding simpler (it abstracts away differences in the video modes) but will be quite slow compared to writing pixels directly to memory.

• I would recommend Borland's Turbo Debugger (TD) for debugging DOS software. Turbo Debugger is included in a number of Borland's DOS C/C++ compiler suites.

Footnotes

• ¹The use of brackets [] in NASM differs from MASM/TASM/JWASM.
• ²Although your question says you want to write to the upper left of the screen, the code suggests you really intended to write the pixel at coordinate 100,100.