I think that high numbers of invalid votes would definitely raise questions about the legitimacy of the elections. Maybe it's a sign that there is a barrier to effective participation and something needs to be amended in the voting process. It may help to differentiate a spoiled ballot and a vote of non-confidence or "none of the above".

A related question to consider would be how low does voter turnout have to fall before elections should be considered illegitimate? This is a question that has crossed my mind while doing research for a working paper on student government elections in Canada (https://www.researchgate.net/publication/369030801_Voter_Turnout_Analysis_of_Canadian_Undergraduate_Student_Unions_2016-2018). In some cases, the turnout was less than 10%.

For potential implications for practice, one potential way forward may be to have mechanisms in place to either have a new election with new candidates or options, or enact another course of action if spoiled ballots or votes of non-confidence surpass a certain percentage of votes cast, or if voter turnout falls under a certain percentage of the electorate.