Tkinter fonts are blury and pixelated

Question

I am trying to make a GUI in Ubuntu 16.04, python 3.6 running the code using virtual environment. I have many default fonts installed on my system.

No matter what code I try my GUI can't seem to use them. The fonts looks really bad, blurry and pixelated:

The list of fonts that the GUI keep using are only these:

I tried to set the fonts using:

style.configure('TLabelframe.Label',background='#f5f6fa',font=("Helvetica",14))

and

tab_Font = Font(family="Lato", size=13)

When I tried this code:

print(Font(font='Lato 13').actual())

It displays:

{'family': 'fixed', 'size': 13, 'weight': 'normal', 'slant': 'roman', 'underline': 0, 'overstrike': 0}

which means fixed font is used instead of Lato.

I have fonts in /usr/share/fonts. I also have fonts in /home/minhaj/.fonts so that's not the problem of fonts not installed.

I simply want to use some decent fonts so that my application doesn't look that bad.