Learn Portuguese (Portugal): O tempo muda muito no outono. - The weather changes a lot in autumn.

Yes! In Portuguese, o tempo means both "time" (like hours passing) and "weather" (like rain or sunshine). You just have to look at the context to know which one it is.

Why is it no outono and not just em outono?

In English, we say "in autumn", but Portuguese requires you to use "in the autumn". You combine the preposition em (in) with the masculine definite article o (the) to get no. You will do this for all seasons, such as no verão (in summer) and na primavera (in spring).

Why doesn't muito change its ending here?

Because it is describing how much the weather changes, making it an adverb. When muito modifies a verb like muda, it stays exactly as it is. It only changes its ending to agree with nouns, like if you were to say muita chuva (a lot of rain).