My first butterfly shot

Well, let me take a crack at it.
It's a pretty good effort for a first try. you've good separation from your background and nice colors, pretty good composition in general. I'd crop some of the top, down to where the flowers start, as the empty space adds little to the image.
Changing the ISO - by itself - won't improve the sharpness. In fact, going with a lower ISO usually means better detail with less noise. Higher ISO would however let you use a faster shutter speed without depending on SSS, which should improve sharpness by reducing motion blur/camera shake, etc. If you're on a tripod, you want to turn off your SSS or it could reduce sharpness.
You could also use a different lens for this kind of work. Zoom lenses have improved a lot over the years, but you've got yours racked out to the extreme - probably trying to get closer - and no zoom lens is at it's best like that. The folks who excel and this are almost always using prime lenses with a combination of focal length - long enough to allow some working distance so as not to scare the bug - but with a short minimum focus distance - MFD - so they can focus as close as the bug will let them get. The ideal are long true macro lenses - which are usually very sharp - and have very short MFDs.
Alternatively, there are gadgets like close up lenses that screw on the front of the lens like a filter and provide magnification; or extension tubes that shorten you MFD for longer lenses (but also cost you your infinity focus).
Best is to practice. Go through the lessons provided on the site. When you see an image you particularly like, ask the photographer for the details, most will be glad to answer a few questions. It's one of the things that makes this site so good.

And welcome aboard   

First, welcome!
Second, it's a nice picture - agreed the top needs cropping and maybe a little on the left, though I'd prefer a little more of the flowers. Tight cropping isn't always best for insects as it can destroy the context. And on my monitor the picture doesn't look badly blurred - the antennae and legs look sharp.

What aperture were you using? Getting all the interesting bits in focus is a challenge in macro pictures, one reason why I often use f11-f16. Looking at the flowers, the ones to the extreme right look sharpest which suggests the lens was focussed a bit in front of the butterfly - have you cropped it heavily on the right?

I find that AF isn't always reliable in macro shots, for a variety of reasons; I like to use MF with focus peaking for real close-ups, though I did actually shoot a lot of butterfly pics two days ago with AF (75-300mm Minolta), mainly at 300mm and about five feet away (they were wild and mostly flew away if I got much closer!). Normally I'd use the 55-200mm which focuses a bit closer (1m), though still not what you'd probably call macro. I generally use a 135mm MF with bellows for real macro stuff. I'm impressed that you managed to get close enough at 50mm!!

As momech says, a faster shutter usually speed helps - much higher with most insects, though butterflies tend to be more static. I feel that the usual rule (shutter speed at least equal to focal length) doesn't apply in macro shots - the greater the magnification, the more effect camera shake has for a given focal length.

I appreciate that light levels were low and this limits your options - I generally keep the aperture smallish and let the ISO rise up as it needed. Usually pics are okay up to 1000 or so ISO and get pretty grainy over 2000.