Depends on what you consider "too." The temperature and percipitation charts included with the Green Mountains Loop show, for mid-May, an average daily high of about 66 F and an average nightly low about 45 F. Total average rainfall for the month is slightly over 3". It's higher in June-September.
I did much of the route as a loop from/to Burlington during ACA's supported camping tour, so I can vouch for the pretty scenery. Burlington makes a good starting point as there is a campground at the north end of town. The towen intself was very vibrant. Our loop included the "East Alternative," which went through some classic Vermont countryside. If I recall correctly, camping two nights was somewhat of an issue. I recall havign to go off route. One night was at a private campground (can't remember the name) and another at, I think, Silver Lake State Park. The other nights we stayed at Lake Carmi State Park (night 1), Brighton State Park near Island Pond (night 2) and Button Bay State Park (final night). All camping locations were nice. Glancing at the map, it looks like the main route might have a few more camping options on route compared to the East Alternative. I think the same is true for grocery sources. There were some stretches on the East Alternative that didn't have to much in that respect. For example, I remember nothing on route close to Lake Carmi, but Island Pond did have a decent grocery store for its location. Right near Siver Lake there was a cute, high-end country market that made prepared food. Can't remember if it had a decent selection of groceries. In short, depending on the route, there may be places where you will need to carry groceries for a ways if you plan to cook.
As for difficulty, as noted, I was not carryng gear so it's somewhat tough to make a comparison. There are definitely lots of ups and downs. The East Alternative has longer hills and longer descents, but fewer shorter, steeper ups and downs. In addition, there were three longer, harder climbs, including Richester Gap, which was painful for a good stretch. Middleburry Gap was not that bad until the final mile or so. The third of the long climbs, which takes you up to Jay Peak out of Richford, can be avoided by taking a flat alternative that goes into Quebec and then back into VT. We went clockwise, so we did Jay Peak on day one and then Rochester and Middleburry Gaps on the penultimate day). After Middleburry Gap, the route back to Burlington was not hard at all.
Hope all this helps.
