No, I don't think people consider part-time workers lazy.
If I'm totally honest (and it pains me to say this because I know how important flexible working is to keep women in the workplace), PT staff can be a bit of a pain in the arse for managers because it takes more time to manage multiple part timers than full timers - e.g. if someone is, say, 0.5FTE, they still need the same amount of regular 1:1s, appraisals etc. Having two 0.5FTE staff takes up twice as much of the manager's time as one 1FTE person. Then training tends to be less efficient, as the PT person still needs to attend as much training but proportionately, it takes far more time out of their working hours, and it costs twice as much to train two PT staff as it does one FT person. And there are actually very few part time roles in my experience where there is genuinely nothing for others to pick up on their days off, even if it's pretty minimal.
There are some advantages to having PT staff, too. There is less impact, for example, from one PT person going off sick if there are other PT staff in similar roles. Holiday cover is also less of a headache, and when PT are able to do additional hours to help in busy periods, that can be very helpful.
Of course, the biggest advantage of PT working is that you are able to retain the skills and contributions of valued members of staff who might otherwise seek employment elsewhere that better suits their work-life balance. As a feminist, I'm very keen to support women to stay in the workplace, and so I have always tried my very hardest to support requests for PT working - in fact, I don't think I have ever refused one yet - but if I'm totally honest, my heart often sinks a bit when people ask to reduce their hours. It's a pain that I'm wiling to suck up, because I value my team and want them to have a good work-life balance...but I know that it will create more work for me and push costs up in the longer term.