We don't know of any laws that outlawed public graffitti per se, in the sense that it was forbidden to scratch or paint on surfaces that didn't belong to you. This of course didn't mean that someone couldn't sue you in court for damages (via an actio legis aquilae, a legal action/suit according to the lex aquilia or an actio de dolo), but I know of no such cases. In theory that would open up the possibility for a monetary punishment towards the offender, usually more than the worth of the damage done to add a punitive effect (poena). On the other hand, house-owners had a duty to keep their houses in good repair, so that might include mending such low-level damages from their own expenses since they were taken for granted.

If that ever occurred I cannot say (maybe there are some examples hidden in the depth of Roman legal literature), but there are two things we know:

First, that graffiti were ubiquitous. They were found on any available surface and ranged from political propaganda and advertisements for large games and festivities over graffiti by fans praising gladiators and their heroics to toilet humour and sexual insults. There was a whole group of people that made professional graffiti, whitewashing the walls and painting beautiful letters on it with brushes, often working in teams (we know this from surviving examples from Pompeii).

Second, we know that many people were not happy about having their walls and monuments painted with election propaganda or slurs and all the other stuff that we find still today scribbled on walls. That much hasn't changed. We know this because some people were so unhappy about the state of affairs that they put up signs of their own that tried to discourage would-be graffiti artists from using their property as canvas. Some of these signs were made of more durable material than the graffiti, and so they

One such example has been found in Narni, in Umbria. It looks like it once belonged to a larger monument, but it's hard to say what it was. The inscription reads:

Ita candidatus quod petit fiat tuus / et ita perennes scriptor opus hoc praeteri / hoc si impetro a te felix vivas bene vale [CIL XI 4126]

"So may your candidate become whatever he seeks, and so, writer, pass by this eternal monument! If I gain this from you, may you live happily, farewell!" This inscription seems preventive in intention and so already assumes that whatever monument this was part of would become a likely target for graffiti vandalism.

Another, similar example would be a table from Aquileia, that said:

Ita valeas scriptor hoc monumentum / praeteri [CIL V 1490]

"So let it be good for you, writer, pass by this monument"

Or even more direct (CIL X, 3785):

hanc aram / ne quis dealbet

Translated: "May no one whitewash this altar!"

The great tombs and tombstones lining the main thoroughfares were probably a big target for such graffiti, since they were in such highly visible and visited places. None of those cautionary inscriptions however threaten direct legal action or punitive fines (which they sometimes do in case of other possible threats, like illegal interment on the plot), they seem to rely on discouraging would-be defacers or pleading them to choose another target.

In conclusion, it was probably as 'completely fine' as it is today, meaning it is unlikely that you will face serious trouble, while many people will not like what you do.