Are Large Blisters Common After Laser Therapy?

The legs heal very slowly.  That is a long time to be healing, though.  If you had a ruby laser, I would expect a week or so of your legs looking bad.  But for most lasers, you should be healed. I would have to know your  laser and therapy for an accurate answer.  I would also not rule out that you are having a reaction to the Neosporin and inflamed from the medication.  I would switch to Aquaphor.  Be sure to talk with your doctor about this first.

If appropriate precaution is taken (e.g. perform test area, icing if chilled tip not availabl) , there should not be any blistering or scabbing after laser hair removal. When blistering or scabbing occurs, you should demand to be evaluated  by the supervising physician, start lubricating the open sores and possibly start oral antibiotics to prevent secondary infection. This is illustrative of the need to ascertain the supervising physician of a laser clinic be a board-certified dermatologist or plastic surgeon who is competent to manage post laser complications such as blistering and/or post-treatment pigmentation.

A CO2 laser creates a second degree burn of the face and creates an open wound or blister. The healing process will lead in selected patients to an aesthetically improved appearance. There are negative consequences as well.

Laser vaporizes tissue then relies on the body to heal and repair the tissue with scarring and some regeneration.
Fat grafting does not destroy any tissue and the stem cells in the fat actually rebuild and restore your skin.
The video below explains some of this and shows what can be achieved with fat grafting. I gave up laser some time ago precisely because of cases like yours.

Your experience is not normal. Blisters are not a desired clinical endpoint for any of the laser treatments we provide and we have done 75,000 of them over the past 5 years with 10 different lasers. If your legs were being treated it's possible you were getting hair removal, spider veins or even brown spots. None of these conditions would normally result in the kind of trauma and infection you are describing.
What is just as concerning is the cavalier attitude of the "technician" you describe. Class IV medical lasers are legally only for use by physicians, nurses, nurse practitioners and physician assistants in the state of California. Did the "technician" that performed the treatment have one of those licenses? If not, you should be considering legal action, given the nature of your condition and the attitude of the person that saw you.