I think that most the hike-in lakes are probably going to be snowed-in and iced-over still, but I have not been up in the area lately and am in no way an expert. I have been looking in my hiking books the last couple of days to try and find something along Hwy 2 and most of them say June or July are when they are snow free.
A couple of years back I hiked up the foss trails. We fished them all the way to Big Heart. This was in the beginning of August. There were still patches of snow in places. The fishing was great. There are mostly brooks in the upper lakes. I did hook into a 13 inch rainbow though. I was fishing a burgandy crystall bugger. The fish loved it. If you stay the night, watch out for the bats around dusk. I do not know why but they just mobbed me while I was fishing. I think it is the whistling from the back casting that drives them nuts. Also make sure all your skin is covered. Repellent will keep the skiters off but there is a little black kind of knat that will eat your skin off. I have no idea how to keep them at bay.
Most of the hike-in mountain lakes near Index and on east to Stevens Pass and beyond are still iced over and many of the trails are still snowed in. Depending on their elevation and exposure, a lot of them won't be fishable until June or even July. Check with the USFS ranger station at Skykomish for the latest information.
However, there are a few foothill lakes west of Index nearer to Goldbar and Sultan that should be open and require a short hike (sometimes on a logging road). You'll need a Green Trails or other good topographic map to find them.