Translation:It is too hot in the sun. There is a tree over there.
I could be wrong, but I get the feeling that when I say "on liian kuuma" I generally mean that the situation makes ME feel too hot, not that it's objectively so. Minulla on kuuma (I'm hot) / Minulla on kylmä (I'm cold) are set expressions, so the sentence implies "When I am in the sun, I am feeling too hot".
I searched "on liian kuumaa" (parentheses included) on google and almost all the results were sentences where something was too hot for some particular purpose, e.g. Kaakao on liian kuumaa juotavaksi (The hot cocoa is too hot for drinking). I guess you could say "kaakao on liian kuuma juotavaksi", but it would sound like you're trying to chuck the whole thing in one go.
You could get away with it by saying something like "Auringossa on liian kuumaa piknikille" (It's too hot in the sun for a picnic) so you would be emphasizing the action rather than how the weather makes you feel. "Liian kuuma" makes me think you're talking about how the temperature is affecting you or other living beings.
Summa summarum: it's a subtle difference. I wouldn't say "kuumaa" here, but I wouldn't go out of my way to correct someone who did.