If you have a choice, a few weeks earlier would be better in terms of crowds and likely weather, but these dates aren't bad.
Considering that Monday, May 25 is Memorial day, this is the beginning of busier summer season, so on average it will be somewhat crowded, but not as bad as peak holiday times like 4th of July and christmas week. Although weather is always difficult to predict, on average May tends to be drier than June (which is the rainiest season), and the high temps will average around 88-90 as you approach June. Fortunately in summer the rains tend to be relatively short. Definitely keep track of the weather online as you get very close to your travel date. Please note that 10-day forecasts tend to not be very accurate, wait until you are within a day or two of your departure date, and re-check when you are there as much as possible. If rain is forecasted, I highly recommend you bring some rain ponchos/jackets.
Overall, I think Pop Century is a great choice for a Value Resort for 21 year-olds. The themeing is a bit more adult than the All-Star Resorts, and you are less likely (it still could happen) to have large groups (bands, football teams, etc.) staying at this resort.
The transportation is very simple from Pop Century -- the only option is bus, and there is only one central area to wait for the buses. In general the wait for a bus during peak times shouldn't exceed 20 minutes, but there are some cases where it may be a bit longer. In my experience, the availability and reliability of buses is good at WDW and it is basically the same at all resorts.
Hope that helps.