Technically, it's a little different in each country.  Most people consider ripping your own ISO perfectly legal, but the law in some countries state that digitally copying any copyrighted material is illegal (meaning that putting songs onto your MP3 player is technically illegal).

I don't think anyone would care if you dumped your own UMD into an ISO.  And they won't be investigating you whether you actually own the UMD or not.  So they shouldn't consider this illegal.

Making homebrew or modifying design isn't illegal in any respect (unless it involves copyright circumvention or that type of stuff).

Now, legalities out of the way (chances are that your employers probably wouldn't even know that much about the particular console), whether this will help you get a job also depends.  By all means, if you're applying for a programming job, showing what homebrew you've made to your employers would probably not be a bad idea.
Just a GUI/skin probably won't help you much for a game related position these days, since basically every game is 3D (thus if you want to show off a portfolio, you'll need to include other stuff).