Love this program, but the toolbar is very much 2010.
It needs to come into the current decade by being much nicer visually.
Larger icons across the board definitely. Not sure about the rest, though a larger version like the Ribbon toolbar in the Office suite is a good shout.
I had a look at the Mac version which doesn’t seem to have a toolbar at all but rather the main options as icons.
Also, welcome. 
Just re the ribbon UI idea specifically, thought I'd share some drawbacks from the programs I've used with them:
Decisions about what items are full size (large icon at top, text on bottom) vs compact (space-efficient original scale) are set in stone don’t always suit all users. Simple example: Word's giant Paste button with text but then having small icon-only buttons for copy and cut.
On the other end are programs that make every button full size (see: attached GIF of ScreenToGif's editor), as though to democratize which are prominent, yet ends up feeling very busy and counterintuitively can make quickly finding the right button harder, at least in my experience.
Perhaps most importantly, ribbon UI applications I've used wholly replace the main menus with the associated ribbon tab, rather than being a separate element. They typically also contain far fewer items necessarily from the larger buttons. This comes at the expense of requiring more time to navigate between, not being immediately available unless using Alt hotkeys and not having room for a large number of items. I think this aspect alone would be a turn-off for power users of Mp3Tag.
Collectively these can give the impression of a UI being 'dumbed down'. Though not all programs are feature-dense so it can work in some cases.
I'd add that Mac apps avoid the issue of wholesale menu replacement since they all have a permanent, separately located menu bar.
I had to check that I hadn’t written the OP myself. 2010 indeed, if not even 1995.
The tiny elements of the interface makes mp3tag a grudge use. It has helpful functions but it’s a challenge to use it, and not a welcome one.
I see there’s a thread about changing the font, size in particular - ha! - I see I wrote a post there 2 years ago. Nothing seems to have happened to make the GUI more usable. Remember, it’s a GUI - a graphical user interface, not just a functional facility. It’s a visual method of use, a massive step-change over the command-line method these things began with. The interface induces effective and pleasant interaction and purpose.
I just wrote a similar assessment to Audacity, another one with a very agricultural interface.
Florian - we need your attention - an update without the speedbumps inhibiting contributions to your development costs.
I see you're taking the "request" in "feature request" very seriously.
FYI, changing the font size is on my internal wish list, so is customizing the toolbar buttons. Both are massive development efforts.
As for the toolbar icons, a ribbon toolbar is not going to happen. I understand that some people like it, but I don't. A method to make the icons in the toolbar bigger was added with v3.28, already a year ago, c.f.,
Depending on the screen resolution, the Windows DPI settings, and/or the individual strength of eyesight, the toolbar icons could be perceived as too small and as difficult to identify and distinguish. I’ve added support for scaling toolbar icons up to 200%. You can find the new configuration setting at File → Options → Appearance.