The Indian monsoons that drench eastern Nepal in particular are at their peak in August. For the hardy adventurers that visit Nepal at this time of year, wet weather gear is essential. You'll need to be flexible too as transport is often disrupted by the rains.
It's peak monsoon season in August in Nepal. Pokhara in particular gets 850mm of rain in just one month. It's warm and humid in most parts of the country too, but once the rain showers have arrived and dowsed everything the atmosphere does tend to freshen up a little. There's a lot of cloud cover at this time of year, so you'll be lucky if you get many opportunities to take in the vast views of the lofty Himalayas.
The upside is that there are few other tourists around, so you can explore the tumbledown temples, prayer-flag bedecked stupas and narrow streets of Kathmandu and Bhaktapur in relative peace. Several trekking routes are protected from the rains by the Himalayas, so trekking in the Upper Mustang region and in sections of the Annapurna circuit like the Nar and Phu valleys is perfectly possible. The valleys are in full bloom and you'll have many trails to yourself. However, mountain views are typically obscured by low cloud.
Can you still travel in Nepal in August? Yes, just about. Are there better months to visit? Without question!