Can I upload a song that is not always on tempo?

So I composed a touching dramatic piano piece, that starts slower than it’s normal tempo and slowly comes into tempo. And this part repeats throughout the song, so you could say it’s slightly out of tempo in some places. Is that acceptable?

Imagine the tempo goes something like 90-92-95-99-105-110-117-120 in a matter of 4-5 seconds.

Yes, of course. You should enter “Varies” in the BPM input box if your track includes sections of varying tempo.