The external crystal oscillator and the integrated operational amplifier inside the single-chip microcomputer chip form a Shi Ying crystal oscillator, which outputs a sine wave with a single frequency, so the output of the crystal oscillator is a sine wave, and the waveform you see is correct. Single-chip microcomputer is a digital circuit, and all its signals must be digital, which is probably caused by insufficient understanding of oscillator circuit.

If the single chip microcomputer wants to use it as a digital clock source, it must use the trigger inside the single chip microcomputer to shape it into a square wave.