How to deal with my own sloppy research published when I was an undergrad?
Just don't worry about it, and keep doing your current good work.

It's pretty cool that you were able to publish a paper at all as an undergraduate.  Anybody who notices your old paper will primarily notice that.  The sloppiness will reflect much more on your senior co-authors (who should have been expected to know better, and educate you likewise) than yourself.  And if you didn't have much more experienced co-authors, so much more cool and understandable.

Do good work now, and the sloppiness of your undergraduate paper will simply fade into insignificance.