Effect of metal resistivity on the characteristic impedance of microstrip line
If you are using a microstrip then it's likely that you are dealing with frequencies in the tens of MHz upwards. Let's use 100 MHz as an example and let's take the example of L being 250 nH per metre and C being 100 pF per metre. Together they produce a characteristic impedance of \$\sqrt{250/0.1}\$ = 50 ohms.

So how much is the impedance of 250 nH at 100 MHz - do the sums and you'll get 157 ohms. 

So, if you increase your resistance per metre to something approaching or in excess of this then you are going to see changes to the impedance. It all boils down to how far you increased the resistance per metre and how you measured (or judged) the value of characteristic impedance.

I suspect that you went nowhere near 100 ohms per metre because the resistivity of a copper track 1 metre long would be about 1 ohm or less.