いい天気ですね is a lot like saying “It’s nice outside” and trusting in your listener to figure out that by “it” you mean “today’s weather”.
As a side note, it’s not that uncommon to hear someone say the full 今日はいい天気ですね. Just because you CAN leave out an obvious topic word doesn’t mean you HAVE to leave it out (although the Japanese usually do).
今日 = きょう = today
If You Know What I Mean
Yellow: If は marks the main topic of a sentence, how come some sentences don’t have a は?
Yellow: Like いい天気ですね.
Blue: If your listener already knows what the main topic is going to be you don’t have to actually include it in the sentence.
Blue: So you could say 今日はいい天気ですね.
Blue: But you don’t have to include the 今日は because people know that you’re probably talking about today’s weather.
Yellow: So the better you are at guessing my topics the less 日本語 I have to actually use?
Blue: Don’t use this as an excuse to skip out on memorizing new vocabulary!