In fact, the opinion of the locals is that Riva is a gorgeous place because of the Lake. I always found it slightly odd that being so near so beautiful mountains they actually praise the lake. But they probably have some points:
*) the lake attracts more tourism, beach, sailing, surfing, and generates more wealth
*) In northern Italy you are always near the mountains. But not always a quarter mile from a big beautiful lake.
*) And although the mountains around Riva are great, you only need to drive 1-2 hr to be in the most beautiful mountains in the world. Mt. Baldo is good. But come'on it's not the Marmolada, or the Civetta, or the Lavaredo Peaks or the Sella Group or the Vajolet Towers. So of course, in Riva it's the lake that stand out.