Last year one of my papers was rejected. The editor provided the comments of referees so I have continued to work on this paper following their suggestion. I have done a great deal of work and I think that now the paper is really worth a publication.
I was considering to resubmit it to the same journal: can I do this or is it frowned upon?
Some journals won't accept a resubmission after rejection. For some others it might depend on the reason for rejection. If the editor deemed it not suitable for the journal it isn't likely to be reconsidered.
But otherwise you should be able to resubmit it. You can also ask the editor whether it would be advisable or not. Follow their advice, of course.
And "frowned upon" only matters if the editor is the one frowning.
If you submit a previously-rejected paper to the journal, what happens in principle* is that the editorial management system flags the paper as a duplicate. Screenshot if you want to see what it looks like from the other side.
Once flagged, the editor (or more likely the desk editor, i.e. the employee of the publisher) will have to decide if they should pass the paper on to begin the peer review process. The default reaction will be "no". You will need to convince them otherwise. The easiest way is to write an explanation in the cover letter. There's a good chance they'll be looking at the previous reviewer reports while making this decision, so you might want to address the reports directly.
The worst that can happen is that your paper is desk rejected. Rather more likely (assuming you did a good job updating the paper) is that the paper is sent for review, but the same reviewers that reviewed the original paper are invited again.
tl; dr: yes, you can resubmit.
*This is only in principle. It's possible the journal fails to detect the resubmission.
