It is unorthodox, but it's not bad per se. I'd definitely recommend learning to barré properly/use your first finger too, and then use whichever is appropriate based on the specific chord and its context.
A classical guitarist (or at least, a classically-trained guitarist) wouldn't really use their thumb like that at all, but plenty of top guitarists still do: Metheny, (Jeff) Beck, Emmanuel, Hendrix, SRV, Bonamassa, etc... Loads.
I find it does produce a slightly different sound - somewhat warmer and fluffier than using your first finger. This is because your thumb is going to be quite flat against the neck, whereas your first finger would be more pointed (i.e. you would be using your fingertip more).
There are no downsides to this technique if applied appropriately. However, if used as an excuse to not learn other fingerings, you might find it limiting when playing chords that demand a full barré or similar.
So yes, it's fine, but learn to play it both ways and then use whichever you feel.